What do the g, i, m, s, u flags mean?

g (global) finds every match instead of stopping at the first one; i makes matching case-insensitive; m makes ^ and $ match the start/end of each line instead of just the whole string; s (dotAll) makes . match newline characters too; u enables full Unicode mode, affecting how character classes and surrogate pairs are interpreted.

Why does my pattern only show one match without the 'g' flag?

By default, JavaScript's RegExp.exec stops after the first match. Enable the 'g' (global) flag to find every occurrence in the test string instead of just the first one.

What are capture groups and why do they matter?

Parentheses in a pattern, like (\d+), create a capture group that extracts just that portion of the match separately from the full match. This tool lists each capture group's value below its parent match, which is essential for patterns used to extract specific pieces of data like a date's year, month, and day.

Why do I get an 'invalid pattern' error?

JavaScript's regex engine rejects syntactically invalid patterns — common causes are an unclosed parenthesis or bracket, an invalid quantifier like a{,} , or a backreference to a group that doesn't exist. The error message shown comes directly from the browser's RegExp constructor.

Is this JavaScript regex or PCRE/Python regex?

This tool uses JavaScript's native RegExp engine (ECMAScript regex syntax), which is very close to but not identical to PCRE (used by PHP, and similar to Python's re module) — most patterns are compatible, but some advanced features like PCRE's recursive patterns or possessive quantifiers aren't supported in JavaScript.

Does this tool send my text to a server to test the regex?

No. Pattern matching runs entirely in your browser using JavaScript's built-in RegExp object. No network request is made — you can verify this in your browser's Network tab.

Why does an empty match cause the match list to grow very fast?

A pattern that can match a zero-length string (like a* against text with no 'a's) combined with the global flag can theoretically loop matching at every position. This tool guards against that by advancing the search position by one character whenever a zero-length match is found, and by capping the total number of matches, so the page never freezes on a pathological pattern.

Can I test multiline text?

Yes. Paste multiline text directly into the test string area. Enable the 'm' (multiline) flag if you want ^ and $ to match the start and end of each individual line rather than only the beginning and end of the entire string.

How do I match a literal special character like a dot or parenthesis?

Escape it with a backslash — for example, \. matches a literal period instead of 'any character', and \( matches a literal opening parenthesis instead of starting a capture group.

Does this support named capture groups?

Yes. JavaScript's RegExp supports named groups using the (? ...) syntax, and modern browsers expose their values on the match result's groups property, though this tool's UI currently lists all capture groups by their numeric index for simplicity.

Regex Tester — Test & Debug Regular Expressions Instantly

Our Regex Tester lets you write a JavaScript-flavored regular expression, toggle flags (global, case-insensitive, multiline, dotAll, unicode), and test it against any sample text in real time. Matches are highlighted directly in the text, a numbered list shows every match with its position and any captured groups, and invalid patterns are caught with a clear error message instead of crashing. Everything runs locally using the browser's native RegExp engine — no text is ever sent to a server.

Regular expression

//g

Flags

Test string

Highlighted matches

Enter a pattern and test string to see matches

Matches

No matches found

How to Use the Regex Tester — Free Online Regular Expression Tool

1
Type your regular expression pattern into the pattern field (without the surrounding slashes).
2
Toggle the flags you need: g (global — find all matches), i (case-insensitive), m (multiline), s (dotAll), u (unicode).
3
Paste your test string into the text area below.
4
Matches highlight instantly in the text, and a detailed list below shows each match's position and capture groups.

Why Use Regex Tester — Free Online Regular Expression Tool?

Regular expressions are a powerful but notoriously easy-to-get-wrong way to match patterns in text — used for input validation, search-and-replace, log parsing, and data extraction across nearly every programming language. A small typo in a pattern can silently match too much, too little, or throw a syntax error, and debugging that by repeatedly running code is slow. This tool provides an instant feedback loop: type a pattern, see every match highlighted in your sample text in real time, inspect exactly what each capture group extracted, and get a readable error message the moment the pattern becomes invalid — for example, an unclosed group or an invalid quantifier. It uses JavaScript's native RegExp engine directly (the same one powering String.match, String.replace, and RegExp.test in any JS codebase), so a pattern that works here behaves identically once you paste it into your actual code, unlike tools built on a different regex flavor like PCRE or POSIX that can behave subtly differently.

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