Regex Tester

test() returns a boolean — true if the regex matches anywhere in the string. exec() returns the match object (or null) including the full match, capture groups, and index. For global regexes, exec() must be called in a loop to get all matches; test() just tells you if any match exists.

When you call regex.exec() in a loop with a global regex, the regex's lastIndex advances after each match. If a match has zero length (e.g., /a*/ matching an empty position), lastIndex doesn't advance and the loop runs forever. Always check for zero-length matches and increment lastIndex manually, or use String.matchAll() which handles this automatically.

Named groups use the syntax (?<name>pattern). Instead of accessing captures by index ($1, $2), you use the name: $<name> in replacement strings or match.groups.name in code. They make complex patterns much more readable and are supported in modern JavaScript (ES2018+).

Lookaheads and lookbehinds are zero-width assertions. Positive lookahead (?=...) matches a position followed by the pattern. Negative lookahead (?!...) matches a position NOT followed by it. Lookbehinds (?<=...) and (?<!...) work the same way but look behind the current position. They don't consume characters — useful for matching context without including it in the result.

Without the u flag, \d only matches ASCII digits 0–9. It will not match Arabic-Indic digits (٠١٢…) or other Unicode numeric characters. With the u flag and \p{Nd} (Unicode property escape), all Unicode decimal digits are matched.