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<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Perl Masterclass — The Swiss Army Chainsaw</title>
<meta property="og:title" content="Perl Masterclass — The Swiss Army Chainsaw">
<meta property="og:description" content="The definitive Perl reference — regex power, text processing, CPAN modules, and one-liners for real-world automation.">
<meta property="og:image" content="../mathsGraph.jpg">
<meta property="og:type" content="article">
<link rel="preconnect" href="https://fonts.googleapis.com">
<link href="https://fonts.googleapis.com/css2?family=Cinzel+Decorative:wght@400;700;900&family=Cinzel:wght@400;600;700&family=JetBrains+Mono:ital,wght@0,300;0,400;0,600;0,700;1,400&family=EB+Garamond:ital,wght@0,400;0,500;1,400&display=swap" rel="stylesheet">
<style>
:root {
--void: #050408;
--void2: #0a0810;
--void3: #100c1a;
--void4: #181224;
--void5: #201830;
--crim: #cc2020;
--crim2: #e83030;
--crim3: #ff5555;
--crim-dim: #280808;
--gold: #c8960a;
--gold2: #f0b820;
--gold3: #ffd060;
--gold-dim: #1e1400;
--toxic: #70e000;
--toxic2: #90ff20;
--tox-dim: #0c1a00;
--ice: #60a8d0;
--ice-dim: #081420;
--violet: #9966cc;
--vio-dim: #140a22;
--bone: #d4c8a8;
--bone2: #a09070;
--bone3: #604840;
--border: rgba(200,150,10,0.2);
--border2: rgba(200,150,10,0.07);
--text: #d8c8b0;
--text2: #7a6850;
--text3: #3a2c1a;
--mono: 'JetBrains Mono', monospace;
--serif: 'EB Garamond', serif;
--display: 'Cinzel Decorative', serif;
--title2: 'Cinzel', serif;
}
*,*::before,*::after{box-sizing:border-box;margin:0;padding:0;}
html{scroll-behavior:smooth;}
body {
background: var(--void);
color: var(--text);
font-family: var(--mono);
font-size: 13.5px;
line-height: 1.75;
min-height: 100vh;
overflow-x: hidden;
}
/* ── PARCHMENT TEXTURE ── */
body::before {
content: '';
position: fixed; inset: 0;
background:
radial-gradient(ellipse 80% 60% at 30% 20%, rgba(200,150,10,0.05) 0%, transparent 60%),
radial-gradient(ellipse 60% 40% at 80% 80%, rgba(204,32,32,0.04) 0%, transparent 50%),
radial-gradient(ellipse 100% 80% at 50% 50%, rgba(112,224,0,0.02) 0%, transparent 70%);
pointer-events: none; z-index: 0;
}
body::after {
content: '';
position: fixed; inset: 0;
background-image:
linear-gradient(rgba(200,150,10,0.04) 1px, transparent 1px),
linear-gradient(90deg, rgba(200,150,10,0.025) 1px, transparent 1px);
background-size: 60px 60px, 120px 120px;
pointer-events: none; z-index: 0;
}
/* ── HEADER ── */
header {
position: sticky; top: 0; z-index: 200;
background: rgba(5,4,8,0.97);
backdrop-filter: blur(24px);
border-bottom: 1px solid var(--border);
}
.header-inner {
max-width: 1500px; margin: 0 auto;
padding: 0 1.5rem;
display: flex; align-items: center; gap: 0; height: 54px;
}
.logo {
display: flex; align-items: center; gap: 12px;
flex-shrink: 0; padding-right: 2rem;
border-right: 1px solid var(--border);
margin-right: 0;
}
.logo-perl {
font-family: var(--display);
font-size: 1.2rem; font-weight: 900;
color: var(--crim2); letter-spacing: 0.1em; line-height: 1;
text-shadow: 0 0 30px rgba(204,32,32,0.4);
}
.logo-ver {
font-family: var(--mono);
font-size: 0.58rem; color: var(--text3);
letter-spacing: 0.1em; line-height: 1.3;
}
.logo-motto {
font-family: var(--serif);
font-size: 0.7rem; color: var(--gold);
font-style: italic; letter-spacing: 0.04em;
}
nav {
display: flex; align-items: stretch;
overflow-x: auto; scrollbar-width: none; flex: 1;
}
nav::-webkit-scrollbar { display: none; }
.nav-btn {
background: none; border: none;
border-right: 1px solid var(--border2);
color: var(--text3);
font-family: var(--title2); font-size: 0.6rem;
letter-spacing: 0.12em; padding: 0 1.1rem;
cursor: pointer; white-space: nowrap;
height: 54px; transition: all 0.12s;
text-transform: uppercase; position: relative;
}
.nav-btn::after {
content: ''; position: absolute; bottom: 0; left: 0; right: 0; height: 2px;
background: var(--crim); transform: scaleX(0); transition: transform 0.15s;
}
.nav-btn:hover { color: var(--gold2); }
.nav-btn:hover::after { transform: scaleX(1); background: var(--gold); }
.nav-btn.active { color: var(--crim2); background: var(--crim-dim); }
.nav-btn.active::after { transform: scaleX(1); }
.hdr-badge {
flex-shrink: 0; border-left: 1px solid var(--border);
padding-left: 1.5rem; margin-left: auto;
font-family: var(--title2); font-size: 0.58rem;
color: var(--gold); letter-spacing: 0.1em; display: flex; gap: 1rem; align-items: center;
}
.hdr-badge span { color: var(--text3); }
/* ── LAYOUT ── */
.wrapper { max-width: 1500px; margin: 0 auto; padding: 2rem; position: relative; z-index: 1; }
section { display: none; animation: conjure 0.25s ease; }
section.active { display: block; }
@keyframes conjure { from{opacity:0;transform:translateY(6px)}to{opacity:1;transform:translateY(0)} }
/* ── SECTION HEADER ── */
.section-header { margin-bottom: 2rem; }
.section-eyebrow {
font-family: var(--title2); font-size: 0.6rem;
letter-spacing: 0.2em; text-transform: uppercase;
color: var(--gold); margin-bottom: 0.4rem;
display: flex; align-items: center; gap: 1rem;
}
.section-eyebrow::before { content: '§'; color: var(--crim); font-size: 0.8rem; }
.section-title {
font-family: var(--display); font-size: 2.4rem;
font-weight: 900; letter-spacing: 0.08em; line-height: 1.1;
color: var(--text); margin-bottom: 0.5rem;
}
.section-title em { color: var(--crim2); font-style: normal; text-shadow: 0 0 20px rgba(204,32,32,0.4); }
.section-title .sed { color: var(--gold2); }
.section-title .awk { color: var(--toxic); }
.section-sub { color: var(--text2); font-size: 0.88rem; max-width: 800px; line-height: 1.75; font-family: var(--serif); }
.section-rule { height: 1px; margin: 1.25rem 0 2rem; background: linear-gradient(90deg, var(--crim), var(--gold), rgba(200,150,10,0.1), transparent); }
/* ── GRIDS ── */
.g2 { display:grid; grid-template-columns:1fr 1fr; gap:1.5rem; }
.g3 { display:grid; grid-template-columns:1fr 1fr 1fr; gap:1.25rem; }
.g12 { display:grid; grid-template-columns:1fr 2fr; gap:1.5rem; }
.g21 { display:grid; grid-template-columns:2fr 1fr; gap:1.5rem; }
@media(max-width:980px){.g2,.g3,.g12,.g21{grid-template-columns:1fr}}
/* ── GRIMOIRE CARD ── */
.card {
background: var(--void2); border: 1px solid var(--border);
padding: 1.25rem 1.5rem; position: relative;
transition: border-color 0.15s;
}
.card:hover { border-color: rgba(200,150,10,0.4); }
.card::before {
content: ''; position: absolute; top: 0; left: 0; right: 0; height: 1px;
background: linear-gradient(90deg, var(--crim), transparent);
opacity: 0.4;
}
.card-label {
font-family: var(--title2); font-size: 0.58rem; letter-spacing: 0.18em;
text-transform: uppercase; color: var(--gold); margin-bottom: 0.75rem;
display: flex; align-items: center; gap: 0.5rem;
}
.card-label::before { content: '☽'; color: var(--crim); }
/* ── CODE BLOCKS ── */
.code-wrap { background: rgba(0,0,0,0.7); border: 1px solid var(--border); margin: 0.85rem 0; overflow: hidden; }
.code-header {
display: flex; align-items: center; justify-content: space-between;
background: var(--void3); border-bottom: 1px solid var(--border);
padding: 0.35rem 1rem;
}
.code-fname { font-family: var(--title2); font-size: 0.55rem; color: var(--text2); letter-spacing: 0.1em; }
.code-lang-perl { font-family: var(--title2); font-size: 0.52rem; color: var(--crim2); background: var(--crim-dim); border: 1px solid rgba(204,32,32,0.2); padding: 1px 7px; letter-spacing: 0.08em; }
.code-lang-sed { font-family: var(--title2); font-size: 0.52rem; color: var(--gold2); background: var(--gold-dim); border: 1px solid rgba(200,150,10,0.2); padding: 1px 7px; letter-spacing: 0.08em; }
.code-lang-awk { font-family: var(--title2); font-size: 0.52rem; color: var(--toxic); background: var(--tox-dim); border: 1px solid rgba(112,224,0,0.2); padding: 1px 7px; letter-spacing: 0.08em; }
.code-lang-bash { font-family: var(--title2); font-size: 0.52rem; color: var(--ice); background: var(--ice-dim); border: 1px solid rgba(96,168,208,0.2); padding: 1px 7px; letter-spacing: 0.08em; }
.code-lang-raku { font-family: var(--title2); font-size: 0.52rem; color: var(--violet); background: var(--vio-dim); border: 1px solid rgba(153,102,204,0.2); padding: 1px 7px; letter-spacing: 0.08em; }
pre {
padding: 1rem 1.25rem; overflow-x: auto;
font-family: var(--mono); font-size: 0.78rem; line-height: 1.9;
color: var(--text); tab-size: 2;
scrollbar-width: thin; scrollbar-color: var(--void4) transparent;
}
pre::-webkit-scrollbar { height: 4px; }
pre::-webkit-scrollbar-thumb { background: var(--void4); }
/* ── SYNTAX COLORS ── */
/* Perl */
.pk { color: var(--crim2); font-weight: 600; } /* perl keyword */
.pv { color: var(--crim3); } /* perl sigil var $@% */
.ps { color: #88cc44; } /* perl string */
.pn { color: var(--gold2); } /* perl number */
.po { color: var(--gold); } /* perl operator */
.pr { color: #ff8844; } /* perl regex */
.pc { color: var(--text3); font-style:italic; } /* perl comment */
.pb { color: var(--bone2); } /* perl builtins */
.pp { color: #cc88ff; } /* perl pragma */
.pm { color: #88ddff; } /* perl module */
.psp { color: var(--crim3); font-style:italic; } /* special var */
/* sed */
.sk { color: var(--gold2); font-weight: 600; }
.sc { color: var(--text3); font-style:italic; }
.sr { color: #ffcc44; }
.ss { color: #88cc44; }
/* awk */
.ak { color: var(--toxic); font-weight: 600; }
.av { color: var(--toxic2); }
.ac { color: var(--text3); font-style:italic; }
.as { color: #88cc44; }
/* bash */
.bk { color: var(--ice); }
.bc { color: var(--text3); font-style:italic; }
/* raku */
.rk { color: var(--violet); font-weight: 600; }
.ro { color: #cc88ff; }
.rs { color: #88cc44; }
.rn { color: var(--gold2); }
/* ── CALLOUTS ── */
.callout { border-left: 2px solid; padding: 0.8rem 1rem; margin: 1rem 0; font-size: 0.87rem; line-height: 1.65; font-family: var(--serif); }
.callout strong { display: block; font-family: var(--title2); font-size: 0.58rem; letter-spacing: 0.14em; margin-bottom: 0.3rem; }
.c-crim { border-color: var(--crim); background: rgba(204,32,32,0.06); color: var(--crim3); }
.c-gold { border-color: var(--gold); background: rgba(200,150,10,0.06); color: var(--gold2); }
.c-toxic { border-color: var(--toxic); background: rgba(112,224,0,0.05); color: var(--toxic); }
.c-ice { border-color: var(--ice); background: rgba(96,168,208,0.05); color: var(--ice); }
.c-vio { border-color: var(--violet);background: rgba(153,102,204,0.06);color: var(--violet); }
.c-bone { border-color: var(--bone2); background: rgba(160,144,112,0.05);color: var(--bone); }
/* ── STAT STRIP ── */
.stat-strip {
display: grid; grid-template-columns: repeat(auto-fit,minmax(110px,1fr));
border: 1px solid var(--border); margin-bottom: 2rem; background: var(--void2);
}
.stat { padding: 1rem; text-align: center; border-right: 1px solid var(--border); transition: background 0.12s; }
.stat:last-child { border-right: none; }
.stat:hover { background: var(--void3); }
.stat-num { font-family: var(--display); font-size: 1.8rem; font-weight: 900; line-height: 1; margin-bottom: 0.2rem; }
.stat-label { font-family: var(--title2); font-size: 0.48rem; color: var(--text3); letter-spacing: 0.12em; text-transform: uppercase; }
/* ── TABS ── */
.pills { display: flex; gap: 4px; margin-bottom: 1.5rem; flex-wrap: wrap; }
.pill {
font-family: var(--title2); font-size: 0.52rem; padding: 0.3rem 0.9rem;
cursor: pointer; border: 1px solid var(--border);
background: none; color: var(--text3);
transition: all 0.12s; letter-spacing: 0.1em; text-transform: uppercase;
}
.pill:hover { border-color: var(--gold); color: var(--gold2); }
.pill.active { background: var(--gold-dim); border-color: var(--gold); color: var(--gold2); }
.tab-pane { display: none; }
.tab-pane.active { display: block; }
/* ── COMPARISON ROW ── */
.vs-row {
display: grid; grid-template-columns: 30px 1fr 1fr;
gap: 0; border-bottom: 1px solid var(--border2);
transition: background 0.1s;
}
.vs-row:hover { background: rgba(200,150,10,0.03); }
.vs-row:last-child { border-bottom: 1px solid var(--border); }
.vs-num { padding: 0.5rem 0.5rem; font-family: var(--title2); font-size: 0.5rem; color: var(--text3); display: flex; align-items: flex-start; padding-top: 0.7rem; }
.vs-left { padding: 0.5rem 0.75rem; border-right: 1px solid var(--border2); }
.vs-right { padding: 0.5rem 0.75rem; }
.vs-left pre, .vs-right pre { padding: 0; background: none; border: none; font-size: 0.76rem; line-height: 1.7; color: var(--text2); }
.vs-label {
font-family: var(--title2); font-size: 0.48rem; letter-spacing: 0.12em;
text-transform: uppercase; padding: 0.35rem 0.75rem;
border-bottom: 1px solid var(--border);
}
.vsl-sed { color: var(--gold2); background: var(--gold-dim); }
.vsl-awk { color: var(--toxic); background: var(--tox-dim); }
.vsl-grep { color: var(--ice); background: var(--ice-dim); }
.vsl-find { color: var(--violet); background: var(--vio-dim); }
.vsl-perl { color: var(--crim2); background: var(--crim-dim); }
/* ── TABLES ── */
table { width: 100%; border-collapse: collapse; font-size: 0.82rem; }
th {
font-family: var(--title2); font-size: 0.52rem; letter-spacing: 0.14em;
text-transform: uppercase; color: var(--gold2);
border-bottom: 1px solid var(--border); padding: 0.45rem 0.85rem;
text-align: left; background: var(--void3);
}
td { padding: 0.5rem 0.85rem; border-bottom: 1px solid var(--border2); color: var(--text2); font-family: var(--serif); font-size: 0.9rem; }
tr:last-child td { border-bottom: 1px solid var(--border); }
tr:hover td { background: rgba(200,150,10,0.04); }
td:first-child { color: var(--text); font-family: var(--mono); font-size: 0.78rem; }
td code, p code, li code, span code {
font-family: var(--mono); font-size: 0.8em;
background: var(--void3); border: 1px solid var(--border);
padding: 0 4px; color: var(--crim3);
}
/* ── SEP ── */
.sep { border: none; border-top: 1px solid var(--border); margin: 2rem 0; }
/* ── SPECIAL VAR GRID ── */
.var-grid {
display: grid; grid-template-columns: repeat(auto-fill, minmax(200px,1fr));
gap: 1px; background: var(--border); border: 1px solid var(--border);
margin: 1rem 0;
}
.var-cell { background: var(--void2); padding: 0.5rem 0.85rem; }
.var-name { font-family: var(--mono); font-size: 0.82rem; color: var(--crim3); font-weight: 600; }
.var-desc { font-family: var(--serif); font-size: 0.82rem; color: var(--text2); line-height: 1.4; }
/* ── TEXT ── */
p { margin-bottom: 0.85rem; color: var(--text2); font-family: var(--serif); }
p:last-child { margin-bottom: 0; }
h3 { font-family: var(--display); font-size: 1.1rem; font-weight: 700; letter-spacing: 0.08em; color: var(--text); margin: 1.75rem 0 0.6rem; }
h3:first-child { margin-top: 0; }
h4 { font-family: var(--title2); font-size: 0.58rem; color: var(--text3); letter-spacing: 0.16em; text-transform: uppercase; margin: 1.2rem 0 0.5rem; }
ul,ol { padding-left: 1.5rem; color: var(--text2); font-family: var(--serif); }
li { margin-bottom: 0.35rem; }
strong { color: var(--text); font-weight: 600; }
::-webkit-scrollbar { width: 5px; height: 5px; }
::-webkit-scrollbar-track { background: var(--void2); }
::-webkit-scrollbar-thumb { background: var(--void4); }
/* ── RAKU SPECIAL ── */
.raku-badge {
display: inline-block;
font-family: var(--title2); font-size: 0.52rem;
background: var(--vio-dim); border: 1px solid var(--violet);
color: var(--violet); padding: 1px 8px; letter-spacing: 0.08em;
margin-left: 6px; vertical-align: middle;
}
</style>
</head>
<body>
<nav style="font-family:'DM Mono',monospace;font-size:0.7rem;padding:8px 16px;background:#1c1a16;color:#b8b0a4;border-bottom:1px solid #333;letter-spacing:0.03em;position:sticky;top:0;z-index:9999;">
<a href="../index.html" style="color:#cc4400;text-decoration:none;">KeGG</a>
<span style="color:#555;margin:0 6px;">/</span>
<span style="color:#b8b0a4;">Masterclasses</span>
<span style="color:#555;margin:0 6px;">/</span>
<span style="color:#f2ece0;">Perl Masterclass — The Swiss Army Chainsaw</span>
</nav>
<header>
<div class="header-inner">
<div class="logo">
<div>
<div class="logo-perl">PERL</div>
<div class="logo-ver">5.38 · RAKU · SED · AWK</div>
<div class="logo-motto">There is more than one way to do it</div>
</div>
</div>
<nav>
<button class="nav-btn active" onclick="show('philosophy',this)">PHILOSOPHY</button>
<button class="nav-btn" onclick="show('oneliners',this)">ONE-LINERS</button>
<button class="nav-btn" onclick="show('regex',this)">REGEX</button>
<button class="nav-btn" onclick="show('vssed',this)">vs SED</button>
<button class="nav-btn" onclick="show('vsawk',this)">vs AWK</button>
<button class="nav-btn" onclick="show('vsgrep',this)">vs GREP+FIND</button>
<button class="nav-btn" onclick="show('data',this)">DATA STRUCTURES</button>
<button class="nav-btn" onclick="show('raku',this)">RAKU / PERL 6</button>
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<div class="hdr-badge">
<span>TIMTOWTDI</span>
<span style="color:var(--crim)">Larry Wall · 1987</span>
</div>
</div>
</header>
<div class="wrapper">
<!-- ══════════════════════════════════════
PHILOSOPHY
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<section id="philosophy" class="active">
<div class="section-header">
<div class="section-eyebrow">The Swiss Army Chainsaw</div>
<div class="section-title">PERL — <em>Born</em> From sed & awk</div>
<div class="section-sub">Larry Wall created Perl in 1987 because he needed to generate reports from system logs and found sed too weak and awk too limited. He wanted the best of both — plus a full programming language underneath. Perl absorbed sed's stream editing, awk's field splitting and pattern-action model, shell's interpolation, C's operators, and grew into something that defies easy description. A greybeard's weapon.</div>
</div>
<div class="section-rule"></div>
<div class="stat-strip">
<div class="stat"><div class="stat-num" style="color:var(--crim2)">1987</div><div class="stat-label">Born at NASA JPL</div></div>
<div class="stat"><div class="stat-num" style="color:var(--gold2)">TMTOWTDI</div><div class="stat-label">The Philosophy</div></div>
<div class="stat"><div class="stat-num" style="color:var(--toxic)">3</div><div class="stat-label">Virtues</div></div>
<div class="stat"><div class="stat-num" style="color:var(--ice)">$_</div><div class="stat-label">The "It" Variable</div></div>
<div class="stat"><div class="stat-num" style="color:var(--violet)">-npe</div><div class="stat-label">Sed/Awk Mode</div></div>
<div class="stat"><div class="stat-num" style="color:var(--crim2)">PCRE+</div><div class="stat-label">Regex Engine</div></div>
</div>
<div class="g2">
<div>
<h3>The Three Virtues</h3>
<div class="card">
<div class="card-label">Larry Wall's Programmer Virtues</div>
<p><strong>Laziness</strong> — The quality that makes you go to great effort to reduce overall energy expenditure. Write a program others can use so you don't have to answer their questions. Write functions so you don't repeat yourself.</p>
<p><strong>Impatience</strong> — The anger you feel when the computer is being lazy. This makes you write programs that anticipate your needs, not just react to them.</p>
<p><strong>Hubris</strong> — Excessive pride, the kind that makes you write (and maintain) programs that other people won't want to say bad things about.</p>
</div>
<h3>The Lineage</h3>
<div class="code-wrap">
<div class="code-header"><span class="code-fname">Where Perl's parts came from</span><span class="code-lang-perl">HISTORY</span></div>
<pre><span class="pc"># Perl = sed + awk + sh + C + "kitchen sink"</span>
<span class="pc"># From sed: s/pattern/replacement/flags</span>
<span class="pc"># d, p, q, a, i, y (transliterate)</span>
<span class="pc"># line ranges: 1,5 /start/,/end/</span>
<span class="pc"># -n (suppress output) -i (in-place)</span>
<span class="pc"># From awk: field splitting ($1 $2 $NF)</span>
<span class="pc"># pattern { action } blocks</span>
<span class="pc"># BEGIN { } and END { }</span>
<span class="pc"># associative arrays</span>
<span class="pc"># printf, getline</span>
<span class="pc"># -F field separator</span>
<span class="pc"># From sh: backtick execution `cmd`</span>
<span class="pc"># string interpolation "hello $name"</span>
<span class="pc"># here-docs <<EOF</span>
<span class="pc"># file test operators -f -d -e -r -w</span>
<span class="pc"># From C: control flow (if for while)</span>
<span class="pc"># operators ++ -- += *= ? :</span>
<span class="pc"># stdio concepts</span>
<span class="pc"># Perl's own: context (list vs scalar)</span>
<span class="pc"># references and complex data</span>
<span class="pc"># tied variables, overloading</span>
<span class="pc"># BEGIN/END/UNITCHECK/CHECK/INIT</span>
<span class="pc"># source filters, XS extension</span>
<span class="pc"># AUTOLOAD, DESTROY, tie</span>
<span class="pc"># formats (a whole report language)</span></pre>
</div>
</div>
<div>
<h3>Perl's Special Variables — The Grimoire</h3>
<div class="callout c-crim">
<strong>$_ is the heart of Perl</strong>
The default variable. Nearly every Perl builtin operates on <code>$_</code> when given no argument: <code>print</code>, <code>chomp</code>, <code>chop</code>, <code>length</code>, <code>uc</code>, <code>lc</code>, <code>split</code>, <code>tr///</code>, <code>s///</code>, <code>m//</code>. This is what makes Perl one-liners so compact — you never type the variable name unless you have to.
</div>
<div class="var-grid">
<div class="var-cell"><div class="var-name">$_</div><div class="var-desc">Default input/operation target</div></div>
<div class="var-cell"><div class="var-name">$/</div><div class="var-desc">Input record separator (default "\n"). <code>undef</code> → slurp whole file</div></div>
<div class="var-cell"><div class="var-name">$\</div><div class="var-desc">Output record separator (appended after print)</div></div>
<div class="var-cell"><div class="var-name">$,</div><div class="var-desc">Output field separator (between print args)</div></div>
<div class="var-cell"><div class="var-name">$;</div><div class="var-desc">Subscript separator for multidimensional hash emulation</div></div>
<div class="var-cell"><div class="var-name">$.</div><div class="var-desc">Current line number of last filehandle read</div></div>
<div class="var-cell"><div class="var-name">$!</div><div class="var-desc">OS error string (errno). In numeric context: error number</div></div>
<div class="var-cell"><div class="var-name">$@</div><div class="var-desc">Error from last eval{} block</div></div>
<div class="var-cell"><div class="var-name">$$</div><div class="var-desc">Process ID of running Perl script</div></div>
<div class="var-cell"><div class="var-name">$0</div><div class="var-desc">Name of running program</div></div>
<div class="var-cell"><div class="var-name">$1..$9</div><div class="var-desc">Regex capture groups (reset each match)</div></div>
<div class="var-cell"><div class="var-name">$&</div><div class="var-desc">Last successful match string</div></div>
<div class="var-cell"><div class="var-name">$`</div><div class="var-desc">String before last match (pre-match)</div></div>
<div class="var-cell"><div class="var-name">$'</div><div class="var-desc">String after last match (post-match)</div></div>
<div class="var-cell"><div class="var-name">$+</div><div class="var-desc">Last bracket matched by last regex</div></div>
<div class="var-cell"><div class="var-name">@_</div><div class="var-desc">Subroutine arguments (aliased, not copied)</div></div>
<div class="var-cell"><div class="var-name">@ARGV</div><div class="var-desc">Command-line arguments</div></div>
<div class="var-cell"><div class="var-name">%ENV</div><div class="var-desc">Environment variables (modifiable)</div></div>
<div class="var-cell"><div class="var-name">%INC</div><div class="var-desc">Loaded modules → file paths</div></div>
<div class="var-cell"><div class="var-name">@INC</div><div class="var-desc">Library search path (like Python's sys.path)</div></div>
<div class="var-cell"><div class="var-name">$^W</div><div class="var-desc">Warnings flag (better: use warnings)</div></div>
<div class="var-cell"><div class="var-name">$^O</div><div class="var-desc">Operating system name (linux, darwin, MSWin32)</div></div>
<div class="var-cell"><div class="var-name">$^T</div><div class="var-desc">Program start time (unix epoch)</div></div>
<div class="var-cell"><div class="var-name">$^R</div><div class="var-desc">Last successful embedded code assertion result</div></div>
<div class="var-cell"><div class="var-name">$AUTOLOAD</div><div class="var-desc">Name of the undefined sub that triggered AUTOLOAD</div></div>
</div>
<div class="callout c-gold">
<strong>use English — The Sanity Module</strong>
<code>use English qw(-no_match_vars);</code> gives every special variable a human name: <code>$/</code> → <code>$INPUT_RECORD_SEPARATOR</code>, <code>$.</code> → <code>$NR</code> (like awk's NR), <code>$!</code> → <code>$OS_ERROR</code>. The <code>-no_match_vars</code> avoids the catastrophic performance penalty that <code>$`</code>, <code>$&</code>, <code>$'</code> impose on ALL regex matches in a program.
</div>
</div>
</div>
</section>
<!-- ══════════════════════════════════════
ONE-LINERS
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<section id="oneliners">
<div class="section-header">
<div class="section-eyebrow">Command Line Sorcery</div>
<div class="section-title">THE <em>ONE-LINER</em> ARSENAL</div>
<div class="section-sub">Perl's command-line flags are what make it a sed/awk replacement. <code>-e</code> evaluates code, <code>-n</code> wraps in a line-reading loop (like sed -n or awk's default), <code>-p</code> adds an automatic print, <code>-i</code> edits files in-place, <code>-F</code> sets the field separator. Master these five and you retire your sed and awk scripts.</div>
</div>
<div class="section-rule"></div>
<div class="g2">
<div>
<h3>The Core Flags</h3>
<div class="code-wrap">
<div class="code-header"><span class="code-fname">Flag reference</span><span class="code-lang-perl">PERL FLAGS</span></div>
<pre><span class="pc"># -e 'code' execute code (multiple -e allowed)</span>
<span class="bk">perl</span> <span class="po">-e</span> <span class="ps">'print "Hello\n"'</span>
<span class="pc"># -n wrap in: while(<>) { ...code... }
# reads stdin or files, does NOT print by default</span>
<span class="bk">perl</span> <span class="po">-ne</span> <span class="ps">'print if /error/i'</span> app.log
<span class="pc"># -p wrap in: while(<>) { ...code...; print }
# like sed: always prints $_ after code</span>
<span class="bk">perl</span> <span class="po">-pe</span> <span class="ps">'s/foo/bar/g'</span> file.txt
<span class="pc"># -i[ext] in-place edit (backup with extension)
# -i.bak makes .bak backup
# -i alone: no backup (dangerous but useful)</span>
<span class="bk">perl</span> <span class="po">-i.bak</span> <span class="po">-pe</span> <span class="ps">'s/localhost/prod.db/g'</span> config.ini
<span class="pc"># -F'sep' field separator (implies -an)
# sets $, and splits $_ into @F</span>
<span class="bk">perl</span> <span class="po">-F</span><span class="ps">','</span> <span class="po">-ane</span> <span class="ps">'print $F[0], "\n"'</span> data.csv
<span class="pc"># -a autosplit mode: splits $_ into @F
# (used with -n or -p)</span>
<span class="bk">perl</span> <span class="po">-ane</span> <span class="ps">'print $F[-1], "\n"'</span> <span class="pc"># last field</span>
<span class="pc"># -l[oct] chomp each input line, set $\ to "\n"
# so print automatically adds newline</span>
<span class="bk">perl</span> <span class="po">-lne</span> <span class="ps">'print uc'</span> <span class="pc"># uppercase every line, chomp+print</span>
<span class="pc"># -0[oct] input record separator as octal
# -0777 = slurp (set $/ to undef)
# -00 = paragraph mode (blank line = RS)</span>
<span class="bk">perl</span> <span class="po">-0777</span> <span class="po">-ne</span> <span class="ps">'s/\n\n+/\n/g; print'</span> <span class="pc"># collapse blank lines</span>
<span class="pc"># -s enable $opt_x from -x command line flags</span>
<span class="pc"># -M module use module (like -MData::Dumper)</span>
<span class="bk">perl</span> <span class="po">-MData::Dumper</span> <span class="po">-e</span> <span class="ps">'print Dumper \%ENV'</span>
<span class="bk">perl</span> <span class="po">-MPOSIX</span> <span class="po">-e</span> <span class="ps">'printf "%d\n", POSIX::floor(3.7)'</span>
<span class="bk">perl</span> <span class="po">-MList::Util=sum,max</span> <span class="po">-e</span> <span class="ps">'print sum(1..100)'</span> <span class="pc"># → 5050</span>
<span class="pc"># -w / use warnings warnings</span>
<span class="pc"># -d debugger</span>
<span class="pc"># -c compile-check only (no execute)</span>
<span class="bk">perl</span> <span class="po">-c</span> script.pl</pre>
</div>
</div>
<div>
<h3>BEGIN and END in One-Liners</h3>
<div class="code-wrap">
<div class="code-header"><span class="code-fname">BEGIN/END — awk-style setup and teardown</span><span class="code-lang-perl">PERL</span></div>
<pre><span class="pc"># BEGIN{} runs before the first line is read
# END{} runs after all input is consumed
# These work identically to awk's BEGIN/END</span>
<span class="pc"># Count lines (like wc -l):</span>
<span class="bk">perl</span> <span class="po">-ne</span> <span class="ps">'END{print "$.\n"}'</span>
<span class="pc"># Sum a column (like awk '{sum+=$1} END{print sum}'):</span>
<span class="bk">perl</span> <span class="po">-ane</span> <span class="ps">'$s+=$F[0]; END{print "$s\n"}'</span>
<span class="pc"># Print header and footer around output:</span>
<span class="bk">perl</span> <span class="po">-ne</span> <span class="ps">'
BEGIN { print "=== ERRORS ===\n" }
print if /ERROR/
END { print "=== DONE ===\n" }
'</span> app.log
<span class="pc"># Accumulate then process (can't do this in sed):</span>
<span class="bk">perl</span> <span class="po">-ne</span> <span class="ps">'
push @lines, $_ if /relevant/;
END {
my @sorted = sort { length($a) <=> length($b) } @lines;
print for @sorted;
}
'</span> data.txt
<span class="pc"># Set $/ to slurp the whole file in BEGIN:</span>
<span class="bk">perl</span> <span class="po">-ne</span> <span class="ps">'BEGIN{$/=undef} s/\n/ /g; print'</span> file.txt
<span class="pc"># Better with -0777:</span>
<span class="bk">perl</span> <span class="po">-0777pe</span> <span class="ps">'s/\n/ /g'</span> file.txt
<span class="pc"># Multiple -e flags — readable one-liners:</span>
<span class="bk">perl</span> <span class="po">-ne</span> <span class="ps">''</span> \
<span class="po">-e</span> <span class="ps">'next unless /^ERROR/'</span> \
<span class="po">-e</span> <span class="ps">'chomp; push @e, $_'</span> \
<span class="po">-e</span> <span class="ps">'END { printf "%d errors\n%s\n", scalar @e, join "\n", @e }'</span></pre>
</div>
<h3>The @F Autosplit Array</h3>
<div class="code-wrap">
<div class="code-header"><span class="code-fname">@F — automatic field splitting</span><span class="code-lang-perl">PERL</span></div>
<pre><span class="pc"># -a splits $_ on whitespace into @F (like awk $1 $2 $NF)
# -F'sep' sets the separator (regex or string)</span>
<span class="pc"># Print 2nd and 5th field (like awk '{print $2,$5}'):</span>
<span class="bk">perl</span> <span class="po">-ane</span> <span class="ps">'print "$F[1] $F[4]\n"'</span>
<span class="pc"># Note: @F is 0-indexed; awk's $1 = $F[0] in Perl</span>
<span class="pc"># Last field (like awk '{print $NF}'):</span>
<span class="bk">perl</span> <span class="po">-ane</span> <span class="ps">'print "$F[-1]\n"'</span>
<span class="pc"># CSV parsing with -F',':</span>
<span class="bk">perl</span> <span class="po">-F</span><span class="ps">','</span> <span class="po">-ane</span> <span class="ps">'print "$F[0],$F[2]\n" if $. > 1'</span> data.csv
<span class="pc"># $. > 1 skips header line</span>
<span class="pc"># Tab-separated (like TSV):</span>
<span class="bk">perl</span> <span class="po">-F</span><span class="ps">'\t'</span> <span class="po">-ane</span> <span class="ps">'print join(",", @F)'</span>
<span class="pc"># Multi-char or regex separator:</span>
<span class="bk">perl</span> <span class="po">-F</span><span class="ps">'\s*:\s*'</span> <span class="po">-ane</span> <span class="ps">'print "$F[0]\n"'</span> /etc/passwd
<span class="pc"># Rebuild line with modified fields:</span>
<span class="bk">perl</span> <span class="po">-F</span><span class="ps">','</span> <span class="po">-ane</span> <span class="ps">'
$F[2] *= 1.1; # bump 3rd column by 10%
print join(",", @F)
'</span></pre>
</div>
</div>
</div>
<hr class="sep">
<h3>Power One-Liners — The Nasty Ones</h3>
<div class="g2">
<div>
<div class="code-wrap">
<div class="code-header"><span class="code-fname">File operations and text processing</span><span class="code-lang-perl">PERL</span></div>
<pre><span class="pc"># Delete blank lines:</span>
<span class="bk">perl</span> <span class="po">-ne</span> <span class="ps">'print unless /^\s*$/'</span>
<span class="pc"># Number lines (like nl / cat -n):</span>
<span class="bk">perl</span> <span class="po">-ne</span> <span class="ps">'printf "%4d %s", $., $_'</span>
<span class="pc"># Print lines 5–10 (like sed -n '5,10p'):</span>
<span class="bk">perl</span> <span class="po">-ne</span> <span class="ps">'print if 5..10'</span>
<span class="pc"># .. is the range operator — evaluates to true between matches!</span>
<span class="pc"># Print lines between patterns (like sed '/START/,/END/p'):</span>
<span class="bk">perl</span> <span class="po">-ne</span> <span class="ps">'print if /START/../END/'</span>
<span class="pc"># Reverse lines of a file (like tac):</span>
<span class="bk">perl</span> <span class="po">-e</span> <span class="ps">'print reverse <>'</span>
<span class="pc"># Unique lines without sorting (like awk '!seen[$0]++'):</span>
<span class="bk">perl</span> <span class="po">-ne</span> <span class="ps">'print unless $seen{$_}++'</span>
<span class="pc"># Sort and unique simultaneously:</span>
<span class="bk">perl</span> <span class="po">-e</span> <span class="ps">'my %s; $s{$_}++ for <>; print sort keys %s'</span>
<span class="pc"># Print every Nth line (every 3rd line):</span>
<span class="bk">perl</span> <span class="po">-ne</span> <span class="ps">'print if $. % 3 == 0'</span>
<span class="pc"># Sum all numbers found anywhere in file:</span>
<span class="bk">perl</span> <span class="po">-nle</span> <span class="ps">'$s += $_ for /(\d+\.?\d*)/g; END{print $s}'</span>
<span class="pc"># Word frequency count:</span>
<span class="bk">perl</span> <span class="po">-ne</span> <span class="ps">'$c{lc $_}++ for /(\w+)/g;
END { printf "%6d %s\n", $c{$_}, $_ for sort {$c{$b}<=>$c{$a}} keys %c }'</span>
<span class="pc"># ROT13 (classic):</span>
<span class="bk">perl</span> <span class="po">-pe</span> <span class="ps">'y/A-Za-z/N-ZA-Mn-za-m/'</span>
<span class="pc"># Base64 encode a file:</span>
<span class="bk">perl</span> <span class="po">-MMIME::Base64</span> <span class="po">-0777</span> <span class="po">-ne</span> <span class="ps">'print encode_base64($_)'</span>
<span class="pc"># URL decode a string:</span>
<span class="bk">perl</span> <span class="po">-MURI::Escape</span> <span class="po">-le</span> <span class="ps">'print uri_unescape($ARGV[0])'</span>
<span class="pc"># Extract all URLs from HTML:</span>
<span class="bk">perl</span> <span class="po">-ne</span> <span class="ps">'print "$1\n" while /href="([^"]+)"/gi'</span>
<span class="pc"># JSON one-liners (with JSON::PP which is core):</span>
<span class="bk">perl</span> <span class="po">-MJSON::PP</span> <span class="po">-0777ne</span> <span class="ps">'
my $d = decode_json($_);
print "$_->{name}\n" for @{$d->{users}}
'</span></pre>
</div>
</div>
<div>
<div class="code-wrap">
<div class="code-header"><span class="code-fname">The truly nasty: idiomatic Perl power</span><span class="code-lang-perl">PERL DEEP END</span></div>
<pre><span class="pc"># The Schwartzian Transform — decorate/sort/undecorate
# Sort files by modification time (Perl's fastest way):</span>
<span class="pk">my</span> <span class="pv">@sorted</span> = <span class="pb">map</span> { <span class="pv">$_</span><span class="po">->[</span><span class="pn">0</span><span class="po">]</span> }
<span class="pb">sort</span> { <span class="pv">$a</span><span class="po">->[</span><span class="pn">1</span><span class="po">]</span> <span class="po"><=></span> <span class="pv">$b</span><span class="po">->[</span><span class="pn">1</span><span class="po">]</span> }
<span class="pb">map</span> { <span class="po">[</span><span class="pv">$_</span>, <span class="po">(</span><span class="pb">stat</span> <span class="pv">$_</span><span class="po">)[</span><span class="pn">9</span><span class="po">]]</span> }
<span class="po"><*.log>;</span>
<span class="pc"># Orcish Maneuver — memoize inside sort with ||=</span>
<span class="pb">sort</span> { <span class="po">(</span><span class="pv">$cache</span><span class="po">{</span><span class="pv">$a</span><span class="po">}</span> <span class="po">||=</span> <span class="pb">expensive</span><span class="po">(</span><span class="pv">$a</span><span class="po">))</span>
<span class="po"><=></span>
<span class="po">(</span><span class="pv">$cache</span><span class="po">{</span><span class="pv">$b</span><span class="po">}</span> <span class="po">||=</span> <span class="pb">expensive</span><span class="po">(</span><span class="pv">$b</span><span class="po">))</span> } <span class="pv">@items</span><span class="po">;</span>
<span class="pc"># wantarray — different behavior in list vs scalar context:</span>
<span class="pk">sub</span> <span class="pb">context_aware</span> <span class="po">{</span>
<span class="pk">return</span> <span class="pb">wantarray</span> ? <span class="po">(</span><span class="pn">1</span><span class="po">,</span><span class="pn">2</span><span class="po">,</span><span class="pn">3</span><span class="po">)</span> <span class="po">:</span> <span class="pn">42</span><span class="po">;</span>
<span class="po">}</span>
<span class="pk">my</span> <span class="pv">@list</span> = <span class="pb">context_aware</span><span class="po">();</span> <span class="pc"># (1,2,3)</span>
<span class="pk">my</span> <span class="pv">$scalar</span> = <span class="pb">context_aware</span><span class="po">();</span> <span class="pc"># 42</span>
<span class="pc"># Local $/ for block-scoped slurp:</span>
<span class="pk">my</span> <span class="pv">$content</span> = <span class="pk">do</span> <span class="po">{</span>
<span class="pk">local</span> <span class="psp">$/</span><span class="po">;</span> <span class="pc"># undef $/ in this scope only</span>
<span class="pb">open</span> <span class="pk">my</span> <span class="pv">$fh</span><span class="po">,</span> <span class="ps">'<'</span><span class="po">,</span> <span class="pv">$file</span> <span class="pk">or</span> <span class="pb">die</span><span class="po">;</span>
<span class="po"><</span><span class="pv">$fh</span><span class="po">>;</span> <span class="pc"># slurps entire file</span>
<span class="po">};</span>
<span class="pc"># Typeglob aliasing — the dark art:</span>
<span class="po">*</span><span class="pb">alias</span> <span class="po">=</span> <span class="po">\&</span><span class="pb">original</span><span class="po">;</span> <span class="pc"># alias a sub</span>
<span class="po">*</span><span class="pb">STDOUT</span> <span class="po">=</span> <span class="po">*</span><span class="pb">STDERR</span><span class="po">;</span> <span class="pc"># redirect stdout to stderr</span>
<span class="pk">local</span> <span class="po">*</span><span class="pb">ARGV</span><span class="po">;</span> <span class="pc"># localize the diamond operator</span>
<span class="pc"># String repetition operator x:</span>
<span class="pk">print</span> <span class="ps">"-"</span> <span class="po">x</span> <span class="pn">72</span><span class="po">,</span> <span class="ps">"\n"</span><span class="po">;</span> <span class="pc"># 72 dashes</span>
<span class="pk">my</span> <span class="pv">@zeros</span> <span class="po">= (</span><span class="pn">0</span><span class="po">) x</span> <span class="pn">100</span><span class="po">;</span> <span class="pc"># 100 zeros</span>
<span class="pc"># Hash slice — extract multiple keys at once:</span>
<span class="pk">my</span> <span class="pv">@vals</span> = <span class="pv">@hash</span><span class="po">{</span><span class="ps">qw(name age email)</span><span class="po">};</span>
<span class="pv">@hash</span><span class="po">{</span><span class="ps">qw(x y z)</span><span class="po">}</span> = <span class="po">(</span><span class="pn">1</span><span class="po">,</span><span class="pn">2</span><span class="po">,</span><span class="pn">3</span><span class="po">);</span> <span class="pc"># set multiple keys</span>
<span class="pc"># Grep and map as real functions (not just filters):</span>
<span class="pk">my</span> <span class="pv">@evens</span> = <span class="pb">grep</span> <span class="po">{</span> <span class="pv">$_</span> <span class="po">%</span> <span class="pn">2</span> <span class="po">==</span> <span class="pn">0</span> <span class="po">}</span> <span class="pn">1</span><span class="po">..</span><span class="pn">20</span><span class="po">;</span>
<span class="pk">my</span> <span class="pv">@sq</span> = <span class="pb">map</span> <span class="po">{</span> <span class="pv">$_</span> <span class="po">**</span> <span class="pn">2</span> <span class="po">}</span> <span class="pn">1</span><span class="po">..</span><span class="pn">10</span><span class="po">;</span>
<span class="pc"># sprintf for right-align, zero-pad, format:</span>
<span class="pb">printf</span> <span class="ps">"%*d\n"</span><span class="po">,</span> <span class="pn">10</span><span class="po">,</span> <span class="pv">$n</span><span class="po">;</span> <span class="pc"># right-align in field width 10</span>
<span class="pb">printf</span> <span class="ps">"%-20s %5.2f\n"</span><span class="po">,</span> <span class="pv">$name</span><span class="po">,</span> <span class="pv">$price</span><span class="po">;</span></pre>
</div>
</div>
</div>
</section>
<!-- ══════════════════════════════════════
REGEX
══════════════════════════════════════ -->
<section id="regex">
<div class="section-header">
<div class="section-eyebrow">The Engine of Engines</div>
<div class="section-title">PERL <em>REGEX</em> — The Full Beast</div>
<div class="section-sub">Perl's regex engine is the reference implementation that PCRE was built to emulate. It has features that other regex engines still haven't caught up to: code assertions, recursive patterns, possessive quantifiers, atomic groups, the <code>\K</code> keep operator, and the ability to execute arbitrary Perl code inside a regex match. We're going full depth.</div>
</div>
<div class="section-rule"></div>
<div class="g2">
<div>
<h3>The Modifiers</h3>
<div class="code-wrap">
<div class="code-header"><span class="code-fname">All regex modifiers</span><span class="code-lang-perl">PERL REGEX</span></div>
<pre><span class="pc"># /i case-insensitive</span>
<span class="pc"># /g global (find all matches)</span>
<span class="pc"># /m multiline (^ and $ match line boundaries)</span>
<span class="pc"># /s single-line (. matches \n too)</span>
<span class="pc"># /x extended (whitespace and # comments ignored)</span>
<span class="pc"># /e evaluate replacement as Perl code (s/// only)</span>
<span class="pc"># /r return modified copy, don't modify in-place (5.14+)</span>
<span class="pc"># /a ASCII mode (restrict \w \d \s to ASCII)</span>
<span class="pc"># /u Unicode semantics</span>
<span class="pc"># /l locale semantics</span>
<span class="pc"># /p preserve $`, $&, $' (slower)</span>
<span class="pc"># /n no captures ($1..$9 not set, faster)</span>
<span class="pc"># /xx double-extended: even spaces in char class ignored</span>
<span class="pc"># /x is life-changing for complex patterns:</span>
<span class="pk">if</span> <span class="po">(</span><span class="pv">$email</span> <span class="po">=~</span> <span class="pr">/
\A # start of string
( # capture local part
[^@\s]+ # one or more non-@ non-space
)
@ # literal @
( # capture domain
[^@\s]+ # domain part
\. # literal dot
[a-z]{2,} # TLD
)
\z # end of string
/xi</span><span class="po">) {</span>
<span class="pk">print</span> <span class="ps">"user=</span><span class="pv">$1</span><span class="ps"> domain=</span><span class="pv">$2</span><span class="ps">\n"</span><span class="po">;</span>
<span class="po">}</span>
<span class="pc"># /e: substitute with code evaluation:</span>
<span class="pv">$text</span> <span class="po">=~</span> <span class="pr">s/(\d+)/sprintf "%05d", $1/ge</span><span class="po">;</span>
<span class="pc"># Zero-pads every number in $text to 5 digits</span>
<span class="pv">$text</span> <span class="po">=~</span> <span class="pr">s/\bUC:(\w+)\b/uc($1)/ge</span><span class="po">;</span>
<span class="pc"># "UC:hello" → "HELLO"</span>
<span class="pc"># /ee: double-eval (evaluate result as Perl, then evaluate again)</span>
<span class="pv">$tmpl</span> <span class="po">=~</span> <span class="pr">s/\{\{(\w+)\}\}/$vars{$1}/ge</span><span class="po">;</span>
<span class="pc"># Simple template engine in one line</span>
<span class="pc"># /r: non-destructive substitution (returns copy):</span>
<span class="pk">my</span> <span class="pv">$clean</span> = <span class="pv">$dirty</span> <span class="po">=~</span> <span class="pr">s/[^\w\s]//gr</span><span class="po">;</span>
<span class="pk">my</span> <span class="pv">@clean</span> = <span class="pb">map</span> <span class="po">{</span> <span class="po">(</span><span class="pk">my</span> <span class="pv">$c</span> = <span class="pv">$_</span><span class="po">) =~</span> <span class="pr">s/^\s+|\s+$//gr</span> <span class="po">}</span> <span class="pv">@raw</span><span class="po">;</span>
<span class="pc"># trim every element without destroying originals</span></pre>
</div>
</div>
<div>
<h3>Lookahead, Lookbehind, and Assertions</h3>
<div class="code-wrap">
<div class="code-header"><span class="code-fname">Zero-width assertions — the full set</span><span class="code-lang-perl">PERL REGEX</span></div>
<pre><span class="pc"># ── LOOKAHEAD ─────────────────────────────────────────</span>
<span class="pr">(?=pattern)</span> <span class="pc"># positive lookahead: what follows matches</span>
<span class="pr">(?!pattern)</span> <span class="pc"># negative lookahead: what follows does NOT match</span>
<span class="pc"># Find "foo" only if followed by "bar":</span>
<span class="pv">$s</span> <span class="po">=~</span> <span class="pr">/foo(?=bar)/</span>
<span class="pc"># Number not followed by a dot (integer detection):</span>
<span class="pv">$s</span> <span class="po">=~</span> <span class="pr">/\d+(?!\.)/</span>
<span class="pc"># ── LOOKBEHIND ────────────────────────────────────────</span>
<span class="pr">(?<=pattern)</span> <span class="pc"># positive lookbehind: what precedes matches</span>
<span class="pr">(?<!pattern)</span> <span class="pc"># negative lookbehind: what precedes does NOT match</span>
<span class="pc"># Extract value after "Price: ":</span>
<span class="pv">$s</span> <span class="po">=~</span> <span class="pr">/(?<=Price:\s)\d+\.\d+/</span>
<span class="pc"># ── \K: KEEP — the lookbehind shortcut ─────────────── </span>
<span class="pc"># \K resets start of match — everything before \K is
# matched but NOT included in $& or the capture</span>
<span class="pc"># Extract version number after "version ":</span>
<span class="pv">$s</span> <span class="po">=~</span> <span class="pr">/version\s+\K[\d.]+/i</span>
<span class="pc"># cleaner than: /version\s+([\d.]+)/ with $1</span>
<span class="pc"># Delete everything AFTER a semicolon (keep before):</span>
<span class="pv">$s</span> <span class="po">=~</span> <span class="pr">s/\K;.*$//s</span>
<span class="pc"># ── NAMED CAPTURES ────────────────────────────────────</span>
<span class="pv">$s</span> <span class="po">=~</span> <span class="pr">/(?<year>\d{4})-(?<month>\d{2})-(?<day>\d{2})/</span>
<span class="pk">print</span> <span class="ps">"</span><span class="pv">$+{year}</span><span class="ps">/</span><span class="pv">$+{month}</span><span class="ps">/</span><span class="pv">$+{day}</span><span class="ps">\n"</span><span class="po">;</span>
<span class="pc"># %+ is the named captures hash</span>
<span class="pc"># %- is the named captures hash (allows multiple same-name)</span>
<span class="pc"># ── NON-CAPTURING GROUP ───────────────────────────────</span>
<span class="pr">(?:pattern)</span> <span class="pc"># group without capturing to $1 etc.</span>
<span class="pv">$s</span> <span class="po">=~</span> <span class="pr">/(?:foo|bar)baz/</span> <span class="pc"># match foobaz or barbaz, no capture</span>
<span class="pc"># ── ATOMIC GROUP (possessive) ─────────────────────────</span>
<span class="pr">(?>pattern)</span> <span class="pc"># no backtracking inside (atomic)</span>
<span class="pc"># Prevents catastrophic backtracking on complex patterns</span>
<span class="pv">$s</span> <span class="po">=~</span> <span class="pr">/(?>\w+):/</span> <span class="pc"># grab all word chars, don't give back</span>
<span class="pc"># ── CODE ASSERTIONS: (?{}) and (??{}) ─────────────────</span>
<span class="pc"># (?{ code }): execute code during match, set $^R</span>
<span class="pv">$str</span> <span class="po">=~</span> <span class="pr">/(\d+)(?{ $total += $1 })/g</span><span class="po">;</span>
<span class="pc"># Accumulates into $total DURING the match</span>
<span class="pc"># (??{ code }): interpolate result as regex pattern</span>
<span class="pc"># Match balanced parentheses (classic example):</span>
<span class="pk">my</span> <span class="pv">$balanced</span> = <span class="pr">qr/
\(
(?:
[^()]+ # non-parens
|
(??{ $balanced }) # recursive!
)*
\)
/x</span><span class="po">;</span>
<span class="pv">$str</span> <span class="po">=~</span> <span class="pr">/$balanced/</span><span class="po">;</span> <span class="pc"># matches ((a+(b*c))+(d))</span></pre>
</div>
</div>
</div>
<hr class="sep">
<div class="g2">
<div>
<h3>Global Match in List and Scalar Context</h3>
<div class="code-wrap">
<div class="code-header"><span class="code-fname">Context-dependent /g behavior</span><span class="code-lang-perl">PERL</span></div>
<pre><span class="pc"># /g in LIST context: return all matches at once</span>
<span class="pk">my</span> <span class="pv">@nums</span> = <span class="pv">$s</span> <span class="po">=~</span> <span class="pr">/(\d+)/g</span><span class="po">;</span>
<span class="pc"># /g in SCALAR context: iterate (like Python re.finditer)</span>
<span class="pk">while</span> <span class="po">(</span><span class="pv">$s</span> <span class="po">=~</span> <span class="pr">/(\w+)=(\w+)/g</span><span class="po">) {</span>
<span class="pk">print</span> <span class="ps">"</span><span class="pv">$1</span><span class="ps"> => </span><span class="pv">$2</span><span class="ps">\n"</span><span class="po">;</span>
<span class="pc"># pos($s) holds current position in string</span>
<span class="po">}</span>
<span class="pc"># pos() and \G anchor:</span>
<span class="pc"># \G matches at pos() — where last /g match left off</span>
<span class="pk">while</span> <span class="po">(</span><span class="pv">$s</span> <span class="po">=~</span> <span class="pr">/\G\s*(\w+)/g</span><span class="po">) {</span>
<span class="pk">print</span> <span class="ps">"token: </span><span class="pv">$1</span><span class="ps"> at pos=</span><span class="po">"</span><span class="po">,</span> <span class="pb">pos</span><span class="po">(</span><span class="pv">$s</span><span class="po">),</span> <span class="ps">"\n"</span><span class="po">;</span>
<span class="po">}</span>
<span class="pc"># Captures with /g in list context:</span>
<span class="pk">my</span> <span class="pv">%pairs</span> = <span class="pv">$s</span> <span class="po">=~</span> <span class="pr">/(\w+)=(\w+)/g</span><span class="po">;</span>
<span class="pc"># Captures alternate as key,value pairs → direct into hash</span>
<span class="pc"># tr/// — transliterate (not a regex but critical):</span>
<span class="po">(</span><span class="pk">my</span> <span class="pv">$count</span> = <span class="pv">$s</span><span class="po">)</span> <span class="po">=~</span> <span class="pr">tr/aeiou//</span><span class="po">;</span> <span class="pc"># count vowels</span>
<span class="pv">$s</span> <span class="po">=~</span> <span class="pr">tr/a-z/A-Z/</span><span class="po">;</span> <span class="pc"># uppercase (like y///)</span>
<span class="pv">$s</span> <span class="po">=~</span> <span class="pr">tr/\n//</span><span class="po">;</span> <span class="pc"># count newlines</span>
<span class="pv">$s</span> <span class="po">=~</span> <span class="pr">tr/ //s</span><span class="po">;</span> <span class="pc"># squeeze runs of spaces</span>
<span class="pv">$s</span> <span class="po">=~</span> <span class="pr">tr/a-zA-Z0-9//cd</span><span class="po">;</span> <span class="pc"># delete non-alphanumeric (c=complement, d=delete)</span></pre>
</div>
</div>
<div>
<h3>Possessive Quantifiers & Catastrophic Backtracking</h3>
<div class="code-wrap">
<div class="code-header"><span class="code-fname">Possessive + Conditional patterns</span><span class="code-lang-perl">PERL</span></div>
<pre><span class="pc"># Possessive quantifiers (Perl 5.10+): never backtrack</span>
<span class="pr">++</span> <span class="pc"># possessive * (zero or more, no backtrack)</span>
<span class="pr">?+</span> <span class="pc"># possessive ? (zero or one, no backtrack)</span>
<span class="pr">++</span> <span class="pc"># possessive + (one or more, no backtrack)</span>
<span class="pr">{2,5}+</span> <span class="pc"># possessive counted (no backtrack)</span>
<span class="pc"># ".*+" consumes everything, never gives back
# Prevents catastrophic backtracking on long strings:</span>
<span class="pv">$s</span> <span class="po">=~</span> <span class="pr">/^(?:[^"]++|"[^"]*+")*$/</span> <span class="pc"># fast CSV-like check</span>
<span class="pc"># Conditional pattern (?(cond)yes|no):</span>
<span class="pv">$s</span> <span class="po">=~</span> <span class="pr">/(")?(?(1)[^"]+"|[^,]+)/</span>
<span class="pc"># If a " was captured: match quoted field
# Otherwise: match unquoted field</span>
<span class="pc"># Backreference in conditional:</span>
<span class="pv">$s</span> <span class="po">=~</span> <span class="pr">/^(\[)?(?(1)[^\]]+\]|[^[\]]+)$/</span>
<span class="pc"># Named backreference:</span>
<span class="pv">$s</span> <span class="po">=~</span> <span class="pr">/(?<q>["'])(?<body>.+?)\k<q>/</span>
<span class="pc"># \k<name> backreference to named capture
# Matches either "quoted" or 'quoted' correctly</span>
<span class="pc"># Embedded code for complex validation:</span>
<span class="pv">$s</span> <span class="po">=~</span> <span class="pr">/(\d{1,3}(?:\.\d{1,3}){3})(?{
my @parts = split /\./, $1;
die "not IPv4" if grep { $_ > 255 } @parts;
})/x</span><span class="po">;</span>
<span class="pc"># Regex stored in variables (qr// — compiled regex object):</span>
<span class="pk">my</span> <span class="pv">$ipv4</span> = <span class="pr">qr/(?:\d{1,3}\.){3}\d{1,3}/</span><span class="po">;</span>
<span class="pk">my</span> <span class="pv">$date</span> = <span class="pr">qr/\d{4}-\d{2}-\d{2}/</span><span class="po">;</span>
<span class="pv">$log</span> <span class="po">=~</span> <span class="pr">/$date\s+$ipv4/</span><span class="po">;</span> <span class="pc"># compose them</span>
<span class="pc"># split() is regex-powered:</span>
<span class="pk">my</span> <span class="pv">@f</span> = <span class="pb">split</span> <span class="pr">/\s*,\s*/</span><span class="po">,</span> <span class="pv">$s</span><span class="po">;</span> <span class="pc"># split on comma + optional spaces</span>
<span class="pk">my</span> <span class="pv">@f</span> = <span class="pb">split</span> <span class="po">/</span><span class="pr">(?<=\d)(?=[A-Z])</span><span class="po">/,</span> <span class="pv">$s</span><span class="po">;</span> <span class="pc"># split between digit and uppercase</span>
<span class="pk">my</span> <span class="pv">@f</span> = <span class="pb">split</span> <span class="ps">' '</span><span class="po">,</span> <span class="pv">$s</span><span class="po">;</span> <span class="pc"># magic: like awk (trim + split on whitespace)</span>
<span class="pk">my</span> <span class="pv">@f</span> = <span class="pb">split</span> <span class="pr">/,/</span><span class="po">,</span> <span class="pv">$s</span><span class="po">,</span> <span class="pn">5</span><span class="po">;</span> <span class="pc"># max 5 fields</span></pre>
</div>
</div>
</div>
</section>
<!-- ══════════════════════════════════════
vs SED
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<section id="vssed">
<div class="section-header">
<div class="section-eyebrow">Stream Editor Replacement</div>
<div class="section-title">PERL <em>vs</em> <span class="sed">SED</span></div>
<div class="section-sub">sed — the Stream EDitor — processes text line by line with a tiny set of commands: substitute, delete, print, insert, append, quit, address ranges. Perl does everything sed does, plus has variables, arrays, hashes, and full programming. Larry Wall explicitly designed Perl to make sed's use cases trivial.</div>
</div>
<div class="section-rule"></div>
<div class="g2">
<div>
<h3>sed Fundamentals</h3>
<div class="code-wrap">
<div class="code-header"><span class="code-fname">sed command structure</span><span class="code-lang-sed">SED</span></div>
<pre><span class="sc"># sed 'address command' file</span>
<span class="sc"># address: line number, /regex/, or range</span>
<span class="sc"># command: s (sub), d (delete), p (print),</span>
<span class="sc"># q (quit), a (append), i (insert),</span>
<span class="sc"># y (transliterate), = (print line no)</span>
<span class="sc"># Substitute:</span>
<span class="sk">sed</span> <span class="ss">'s/foo/bar/'</span> <span class="sc"># first occurrence per line</span>
<span class="sk">sed</span> <span class="ss">'s/foo/bar/g'</span> <span class="sc"># global (all occurrences)</span>
<span class="sk">sed</span> <span class="ss">'s/foo/bar/i'</span> <span class="sc"># case-insensitive (GNU sed)</span>
<span class="sk">sed</span> <span class="ss">'s/foo/bar/2'</span> <span class="sc"># 2nd occurrence only</span>
<span class="sk">sed</span> <span class="ss">'s/foo/bar/gp'</span> <span class="sc"># replace + print (with -n)</span>