`Regex`

Range

The period . allows selecting any character, including special characters and spaces.
Character Sets [abc].
Negated Character Sets [^abc].
Letter Range [a-z]
Number Range [0-9]

Asterisk *: We put an asterisk * after a character to indicate that the character may either not match at all or can match many times.
Plus Sign +: To indicate that a character can occur one or more times, we put a plus sign + after a character.
Question Mark ?: To indicate that a character is optional, we put a ? question mark after a character.
To express a certain number of occurrences of a character, we write curly braces {n} along with how many times we want it to occur at the end. {n,} for at least how many times we want it to occur. {x,y} to express the occurrence of a character in a certain number range.

We can group an expression and use these groups to reference or enforce some rules. To group an expression, we enclose () in parentheses.
Referencing a Group: with text: ha-ha,haa-haa and pattern (ha)-\1,(haa)-\2 - The first group is used by writing \1 to avoid rewriting. Here 1 denotes the order of grouping. Type \2 at the end of the expression to refer to the second group.
Parentheses (?: ): Non-capturing Grouping - You can group an expression and ensure that it is not captured by references.

Pipe Character | : Alternation - It allows to specify that an expression can be in different expressions. Thus, all possible statements are written separated by the pipe sign |. This differs from charset [abc], charsets operate at the character level. Alternatives are at the expression level.
Escape Character \: There are special characters that we use when writing regex. {}[]/\+*.$^|? Before we can select these characters themselves, we need to use an escape character \.
Caret Sign ^: Selecting by Line Start.
Dollar Sign $: Selecting by End of Line.
Word Character \w: Letter, Number and Underscore.
Except Word Character \W: is used to find characters other than letters, numbers, and underscores.
Number Character \d: is used to find only number characters.
Except Number Character \D: is used to find non-numeric characters.
Space Character \s: is used to find only space characters.
Except Space Character \S: is used to find non-space characters.

Positive Lookahead pattern(?=matching_pattern) - asserts that the pattern only matches if it is followed by a certain pattern. Example: cat(?=\sfish) matches cat only if it is immediately followed by fish, but cat is not included in the final match.
Negative Lookahead pattern(?!non_matching_pattern) - asserts that the pattern only matches if it is NOT followed by a certain pattern. Example: cat(?!\sfish) matches cat only if it is NOT immediately followed by fish..
Positive Lookbehind (?<=matching_pattern)pattern - asserts that the pattern only matches if it is preceded by a certain pattern. Example (?<=black )cat matches cat only if it is preceded by the word black, but black is not included in the final match.
Negative Lookbehind (?<!non_matching_pattern)pattern - asserts that the pattern only matches if it is NOT preceded by a certain pattern. Example (?<!black )cat - select only the number values that not preceded by $.

Flags change the output of the expression. That's why flags are also called modifiers.

Global Flag: The global flag causes the expression to select all matches. If not used it will only select the first match. Example /\w+\.com/g.
Multiline Flag: Regex sees all text as one line. But we use the multiline flag to handle each line separately. In this way, the expressions we write according to the end of the linework separately for each line. Example /\w+\.com$/gm.
Case-insensitive Flag: In order to remove the case-sensitiveness of the expression we have written, we must activate the case-insensitive flag. Example /\w+\.com$/gmi.
Greedy Matching: Regex does a greedy match by default. This means that the matchmaking will be as long as possible. Check out the example below. It refers to any match that ends in r and can be any character preceded by it. But it does not stop at the first letter r. Example: text ber beer beeer beeeer and pattern /.*r/.
Lazy Matching: Lazy matchmaking, unlike greedy matching, stops at the first matching. For example, in the example below, add a ? after * to find the first match that ends with the letter r and is preceded by any character. It means that this match will stop at the first letter r. Example: text ber beer beeer beeeer and pattern /.*?r/.