Test a Regular Expression

What is Regex Tester?

Test a regular expression against sample text with live match highlighting, capture-group inspection, and flag toggles (g, i, m, s, u, y). Useful for prototyping a pattern before pasting it into code, debugging a regex that isn't matching as expected, or teaching someone how a particular expression behaves.

How do I use Regex Tester?

1. Enter your regex in the pattern field and set flags.
2. Paste sample text into the input panel.
3. Matches are highlighted live; expand a match to see named + indexed capture groups.

Regex Tester by the numbers

Engine

Native RegExp (V8 / SpiderMonkey)

Flags

g, i, m, s, u, y

Named groups

Supported (?<name>...)

Replace preview

Yes, with $1 / $<name> refs

Max haystack size

~1 MB comfortably

Common use cases for Regex Tester

• Validating an email-address pattern against a fixture list.
• Extracting every URL from a block of prose.
• Debugging a parser for log-file timestamps.
• Converting a Python regex to JavaScript by testing edge cases.
• Building a redaction rule for phone numbers in user content.

Common pitfalls and how to avoid them

• Backreferences inside lookbehind fail — JavaScript regex does not support them. Restructure using a lookahead or a two-pass replace.
• Unicode characters do not match — Add the u flag and use \p{Letter} / \p{Number} property escapes instead of ASCII-only classes.
• Greedy pattern consumes too much — Switch to lazy quantifiers (*? and +?) or tighten the character class inside the group.

When should I use Regex Tester?

Regex Tester is for JavaScript regex semantics (same engine your app uses). For a language-specific engine (Python, Go, PCRE), behaviours differ on edge cases; prototype here, then verify in your language.