What does the g flag do in regex?

The g (global) flag finds all matches in the string, not just the first. Without g, JavaScript stops after one match. Use g for search-and-replace across the whole text and for counting occurrences.

How do I match any character including newlines?

By default, dot (.) does not match newline characters. Add the s (dotAll) flag so . matches newlines, or use [\s\S] or explicit patterns for multiline text. Enable s in our regex tester flags row when working with multi-line logs or HTML.

What is a capture group?

Parentheses create a capture group that stores the matched substring for reuse. In replace, $1 refers to the first group. Our Groups tab lists each capture so you can debug numbered references in complex patterns.

How do I make regex case insensitive?

Add the i flag so A and a match interchangeably. In JavaScript you can also use /pattern/i syntax. Combine with g for global case-insensitive search: /pattern/gi.

What is the difference between * and + in regex?

Asterisk (*) means zero or more of the preceding element; plus (+) means one or more. ab* matches a, ab, abb; ab+ requires at least one b after a. Use * for optional repetition; + when at least one occurrence is required.

Regex Tester — Free Online Regular Expression Debugger

Regex Tester: Debug Regular Expressions in Your Browser

Test JavaScript regular expressions with live match highlighting, capture group inspection, replace preview, and a library of common patterns. Debug regex before pasting into code, validators, or search-and-replace workflows.

Key Features

Real-time match highlighting with first-match emphasis
Flags: global, ignore case, multiline, dotAll, unicode
Matches list with index ranges and one-click copy
Capture groups table with named group support
Replace preview with $1, $2 backreferences

Common Use Cases

Validate email, URL, and date patterns before production
Debug failing regex from log parsers or form validators
Test replace rules for bulk text transformation
Learn regex syntax with instant visual feedback
Compare match counts across pattern variations

Benefits

Uses the same RegExp engine as your browser JavaScript
100ms debounce keeps typing smooth on large test strings
Common pattern library loads proven regex in one click
Completely private — patterns may contain sensitive data

Why test regex online?

Regular expressions are powerful but easy to get wrong. A live tester lets you see matches, capture groups, and replacement output instantly — without writing a script or refreshing a page.

This tool runs entirely in your browser using JavaScript's RegExp engine. Your patterns and test strings never leave your device.

Flags and replace mode

Toggle g (global), i (case-insensitive), m (multiline), s (dotall), and u (unicode) flags to match how your regex runs in production code.

Use the Replace tab with $1, $2, and named backreferences to preview substitutions before pasting them into your application.

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Regex library