Regex Tester

1. Enter Your Pattern: Type your regular expression in the pattern field between the forward slashes. Escape special characters with backslashes (e.g., \. to match a literal period, \\ to match a backslash).
2. Configure Flags: Enable "g" (global) to find all matches rather than stopping at the first. Use "i" for case-insensitive matching, "m" for multiline mode where ^ and $ match line boundaries, and "s" to make the dot match newlines.
3. Paste Test Text: Enter the string you want to match against in the test string textarea. The tool highlights matches in real-time as you type, showing captured groups separately.
4. Analyze Results: Review the match count, highlighted matches, and capturing group contents. Click any common pattern from the examples to load it instantly and see how it works.

This tool uses JavaScript's native RegExp engine, which implements a subset of Perl-compatible regular expressions (PCRE). Key supported features include character classes (\d, \w, \s), quantifiers (*, +, ?, {n,m}), anchors (^, $, \b), capturing and non-capturing groups, alternation (|), and lookahead assertions ((?=...) and (?!...)). JavaScript does not support lookbehind assertions in all browsers, possessive quantifiers, or atomic groups.

The regex engine uses backtracking for quantifier matching, which can cause catastrophic performance on patterns with nested quantifiers matching overlapping content (e.g., (a+)+ against a long string of "a"s). For user-supplied patterns in production systems, implement execution timeouts. Unicode support requires the "u" flag for proper handling of surrogate pairs and Unicode property escapes (\p{...}).

• Forgetting to Escape Special Characters: Characters like . * + ? ^ $ { } [ ] \ | ( ) have special meaning. To match them literally, escape with a backslash: \. matches a period, \$ matches a dollar sign.
• Greedy vs. Lazy Quantifiers: By default, .* is greedy and matches as much as possible. For HTML tag matching, <.*> matches from the first < to the last >. Use <.*?> (lazy) to match individual tags.
• Anchor Misunderstanding: Without the "m" flag, ^ and $ match only the start and end of the entire string, not line boundaries. Enable multiline mode for line-by-line matching in multi-line text.

Need pre-built patterns? Browse our Regex Pattern Library for tested expressions for emails, URLs, and more. For understanding complex patterns, the Regex Explainer provides plain-English breakdowns.