How do I test a regex online?

Paste your regex pattern, toggle flags (g/i/m/s/u), and paste a test string. Specway's regex tester runs the pattern with JavaScript's native RegExp, highlights every match in the test string, and lists each match with its index, captured groups, and length. Pure browser — nothing uploads.

What's the difference between JavaScript regex and PCRE?

JavaScript regex (used by this tester) is similar to PCRE but lacks a few features: no possessive quantifiers, no named subroutines (\g{name}), no recursion, and look-behinds need a modern engine. For most patterns the syntax is identical. If your pattern is for Python/PHP/Perl and uses advanced features, validate in those engines too.

What do the regex flags mean?

g (global) — find all matches, not just the first. i (ignore case). m (multiline) — ^ and $ match per line, not just per string. s (dot all) — let . match newlines. u (unicode) — treat the pattern as a unicode regex; required for \u{XXXX} escapes and proper handling of surrogate pairs.

Why is my regex slow / hanging?

Catastrophic backtracking. Patterns like (a+)+ or (a|aa)+ can take exponential time on certain inputs. The fixes: (1) use possessive quantifiers if your engine supports them, (2) use atomic groups (?>...), (3) anchor the pattern, (4) avoid nested unbounded quantifiers. The tester limits to 1000 matches to protect your browser.

How do I escape special characters in regex?

Backslash any of: . * + ? ^ $ ( ) [ ] { } | \ /. So to match a literal dot use \\. and a literal backslash use \\\\. Inside a JS string literal you double the backslashes; in this tester's pattern field you write the regex form directly (no string-literal escaping).

What's a capture group vs a non-capture group?

(foo) captures — the matched text is available as a numbered group. (?:foo) groups for grouping/alternation only, no capture. Use non-capturing when you only care about whether something matched, not what it matched. Captures are how you extract pieces (emails, URLs, dates) from a larger string.

Why does my regex match unexpected things?

Three usual suspects: (1) you forgot the ^ and $ anchors, so the pattern matches anywhere in the string. (2) you used . expecting it to mean 'literal dot' — it means 'any character'. (3) you didn't escape special characters. Test with deliberately wrong inputs to confirm the pattern rejects them, not just accepts the right ones.

How do I test a regex online?

Paste your regex pattern, toggle flags (g/i/m/s/u), and paste a test string. Specway's regex tester runs the pattern with JavaScript's native RegExp, highlights every match in the test string, and lists each match with its index, captured groups, and length. Pure browser — nothing uploads.

What's the difference between JavaScript regex and PCRE?

JavaScript regex (used by this tester) is similar to PCRE but lacks a few features: no possessive quantifiers, no named subroutines (\g{name}), no recursion, and look-behinds need a modern engine. For most patterns the syntax is identical. If your pattern is for Python/PHP/Perl and uses advanced features, validate in those engines too.

What do the regex flags mean?

g (global) — find all matches, not just the first. i (ignore case). m (multiline) — ^ and $ match per line, not just per string. s (dot all) — let . match newlines. u (unicode) — treat the pattern as a unicode regex; required for \u{XXXX} escapes and proper handling of surrogate pairs.

Why is my regex slow / hanging?

Catastrophic backtracking. Patterns like (a+)+ or (a|aa)+ can take exponential time on certain inputs. The fixes: (1) use possessive quantifiers if your engine supports them, (2) use atomic groups (?>...), (3) anchor the pattern, (4) avoid nested unbounded quantifiers. The tester limits to 1000 matches to protect your browser.

How do I escape special characters in regex?

Backslash any of: . * + ? ^ $ ( ) [ ] { } | \ /. So to match a literal dot use \\. and a literal backslash use \\\\. Inside a JS string literal you double the backslashes; in this tester's pattern field you write the regex form directly (no string-literal escaping).

What's a capture group vs a non-capture group?

(foo) captures — the matched text is available as a numbered group. (?:foo) groups for grouping/alternation only, no capture. Use non-capturing when you only care about whether something matched, not what it matched. Captures are how you extract pieces (emails, URLs, dates) from a larger string.

Why does my regex match unexpected things?

Three usual suspects: (1) you forgot the ^ and $ anchors, so the pattern matches anywhere in the string. (2) you used . expecting it to mean 'literal dot' — it means 'any character'. (3) you didn't escape special characters. Test with deliberately wrong inputs to confirm the pattern rejects them, not just accepts the right ones.

Free Tool

Regex Tester (2026)

Test regular expressions with live highlighting, capture groups, and match indices. Toggles g/i/m/s/u flags. Uses native JavaScript RegExp — same engine as your code.

Pattern

//gi

Test string

Matches (2)

Email me at hello@specway.com or support@example.org.
Not an email: alice@.com or @broken.io.

1. hello@specway.com @ 12
groups: [1] hello, [2] specway.com
2. support@example.org @ 33
groups: [1] support, [2] example.org

How the tester works

The pattern is compiled with new RegExp(pattern, flags) — the same engine your JavaScript code uses. Matches are walked with .exec() in a global loop and shown live as you type. The tester capped at 1000 matches to prevent runaway patterns from freezing the page.

Patterns developers write all the time

Email: \b[\w.+-]+@[\w-]+\.[\w.-]+\b
URL: https?://[\w./?=&%~+:#-]+
UUID v4: [0-9a-f]{8}-[0-9a-f]{4}-4[0-9a-f]{3}-[89ab][0-9a-f]{3}-[0-9a-f]{12}
ISO date: \d{4}-\d{2}-\d{2}
Hex color: #[0-9a-fA-F]{3,8}

When NOT to use regex

Parsing HTML, JSON, or any nested structure. Regex is a flat-text engine — it can't count balanced brackets or distinguish "real" tags from text in comments. Use a real parser (DOMParser, JSON.parse, dedicated lib) for anything with structure. Regex is for flat patterns: emails, IDs, log lines, prefix extraction.

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Frequently asked questions

When NOT to use regex