Add \xHH hex escape parsing, fix PSAvoidLongLines, update help text and README escape lists

Author: MariusStorhaug

README.md

@@ -47,7 +47,7 @@ Practical implications of the core schema:
 - Integers and floats parse to their native types using invariant culture.
 - Anything else is a string. Quoted strings (`'...'`, `"..."`) always preserve the string type.
 
-The supported YAML subset covers block-style mappings, block-style sequences, nested structures, single- and double-quoted scalars (with `\n`, `\t`, `\r`, `\\`, `\"` escapes in double quotes), document start (`---`) and end (`...`) markers, and full-line / inline `#` comments. Flow style (`[a, b]`, `{a: 1}`), block scalars (`|`, `>`), anchors, aliases, tags, multi-document streams, and `!!timestamp` are not yet supported.
+The supported YAML subset covers block-style mappings, block-style sequences, nested structures, single- and double-quoted scalars (with `\n`, `\t`, `\r`, `\0`, `\\`, `\"`, `\xHH` escapes in double quotes), document start (`---`) and end (`...`) markers, and full-line / inline `#` comments. Flow style (`[a, b]`, `{a: 1}`), block scalars (`|`, `>`), anchors, aliases, tags, multi-document streams, and `!!timestamp` are not yet supported.
 
 ### Example 1: Parse a YAML string
 

src/functions/private/Expand-YamlDoubleQuoted.ps1

@@ -25,6 +25,18 @@
                 '"' { $null = $sb.Append('"') }
                 '\' { $null = $sb.Append('\') }
                 '0' { $null = $sb.Append([char]0) }
+                'x' {
+                    if ($i + 3 -lt $Text.Length) {
+                        $hex = $Text.Substring($i + 2, 2)
+                        $code = 0
+                        if ([int]::TryParse($hex, [System.Globalization.NumberStyles]::HexNumber, $null, [ref]$code)) {
+                            $null = $sb.Append([char]$code)
+                            $i += 4
+                            continue
+                        }
+                    }
+                    $expanded = $false
+                }
                 default { $expanded = $false }
             }
 

src/functions/public/ConvertFrom-Yaml.ps1

@@ -12,7 +12,7 @@
         - Block-style sequences (- item)
         - Nested structures
         - Scalars: strings, integers, floats, booleans (`true`/`false`), null (`null`/`~`/empty)
-        - Single- and double-quoted strings (with `\n`, `\t`, `\r`, `\\`, `\"` in double quotes)
+        - Single- and double-quoted strings (with `\n`, `\t`, `\r`, `\0`, `\\`, `\"`, `\xHH` in double quotes)
         - Document start (`---`) and end (`...`) markers are tolerated
         - Full-line comments (`#`) and inline comments after values
 
@@ -108,7 +108,9 @@
 
         if ($context.Index -lt $lines.Count) {
             $leftover = $lines[$context.Index]
-            throw "ConvertFrom-Yaml: unexpected content at line $($leftover.Number): '$($leftover.Content)'. The document has trailing content that was not consumed by the parser."
+            $msg = 'ConvertFrom-Yaml: unexpected content at line {0}: ''{1}''.'
+            $msg += ' The document has trailing content that was not consumed.'
+            throw ($msg -f $leftover.Number, $leftover.Content)
         }
 
         if ($NoEnumerate -and $result -is [System.Collections.IList]) {

tests/ConvertTo-Yaml.Tests.ps1

@@ -584,4 +584,15 @@ Describe 'Round-trip ConvertTo-Yaml | ConvertFrom-Yaml' {
         $result['value'] | Should -Be ''
         $result['value'] | Should -BeOfType [string]
     }
+
+    It 'Round-trips control characters via \xHH escapes' {
+        # Format-YamlDoubleQuoted emits \xHH for control chars (e.g. NUL, BEL).
+        # Expand-YamlDoubleQuoted must parse them back correctly.
+        $nul = [string][char]0
+        $bel = [string][char]7
+        $obj = [ordered]@{ nul = $nul; bel = $bel }
+        $result = $obj | ConvertTo-Yaml | ConvertFrom-Yaml -AsHashtable
+        $result['nul'] | Should -Be $nul
+        $result['bel'] | Should -Be $bel
+    }
 }