Regex Cheat Sheet & Live Tester

The 'g' (global) flag makes the regex find all matches in a string, not just the first one. Without 'g', methods like match() stop after the first match. With 'g', exec() advances through all matches and match() returns an array of all matches.

Catastrophic backtracking occurs when a regex engine explores an exponential number of possible paths before determining no match exists. It's common with nested quantifiers like (a+)+. Avoid it by using atomic groups, possessive quantifiers (in supported engines), or rewriting the pattern to remove ambiguity — for example, replacing overlapping alternatives with more specific patterns.

In most engines \d matches any Unicode decimal digit, while [0-9] matches only ASCII digits 0 through 9. In JavaScript with the 'u' flag, \d still matches only [0-9]. If you want strict ASCII digits, [0-9] is clearer and slightly more predictable across engines.

Lookaheads (?=...) and (?!...) assert that a pattern does or doesn't follow the current position without consuming characters. Lookbehinds (?<=...) and (?<!...) do the same for what precedes. For example, \d+(?= dollars) matches a number only when followed by ' dollars', without including ' dollars' in the match.

Regex flavors differ across languages. JavaScript lacks lookbehind in older engines, doesn't support named backreferences in replace in all versions, and has different Unicode handling. Python's 're' module supports possessive quantifiers only via the 'regex' package. Always test your pattern in the target language's environment.

Escape special characters with a backslash: \. matches a literal dot, \* matches a literal asterisk, \( matches a literal parenthesis. The special characters that need escaping are: . * + ? ^ $ { } [ ] | ( ) \ /. Inside a character class [ ], only ] \ ^ - need escaping.