Regex Syntax Quick Reference

Regular expressions are a mini-language for describing text patterns. The core building blocks:

• . — Any character except newline
• \d / \D — Digit / non-digit
• \w / \W — Word character ([A-Za-z0-9_]) / non-word
• \s / \S — Whitespace / non-whitespace
• ^ / $ — Start / end of string (or line in multiline mode)
• * / + / ? — 0+, 1+, 0 or 1 repetitions (greedy by default)
• {n,m} — Between n and m repetitions
• [abc] — Character class; [^abc] negates it
• (group) — Capturing group; (?:group) is non-capturing
• a|b — Alternation: matches a or b

Flags

• g — Global: find all matches, not just the first
• i — Case-insensitive
• m — Multiline: ^/$ match line boundaries
• s — Dotall: . matches newlines too
• u — Unicode mode: enables full Unicode matching including surrogate pairs

Greedy vs. Lazy

Quantifiers (*, +, {n,m}) are greedy by default — they consume as much as possible. Add ? to make them lazy: .*? matches as little as possible. This matters most when matching delimited content like HTML tags: <.+> will match from the first < to the last > in the string, while <.+?> stops at the first closing >.

Lookahead and Lookbehind

(?=...) is a positive lookahead — asserts what follows without consuming it. (?<=...) is a positive lookbehind. Their negative counterparts are (?!...) and (?<!...). These are useful for matching text only when it's preceded or followed by a specific pattern without including that pattern in the match itself.