The tool uses JavaScript's built-in RegExp engine, which follows the ECMAScript specification. It supports lookaheads, named capture groups ((?<n>…)), and Unicode mode (u flag).

g finds all matches. i makes A and a equivalent. m makes ^ and $ match line boundaries. s makes the dot match newlines. u enables full Unicode property escapes.

Use $1, $2, etc. to refer to capturing groups. For named groups defined with (?<n>…), use $<n>. For example, pattern (\d{4})-(\d{2}) with replacement $2/$1 turns 2024-03 into 03/2024.

Common causes: special characters like . + * ? are not escaped; the m flag is needed for ^ and $ to match line boundaries; or the g flag is off so only the first match is shown.

Yes. Paste multi-line text into the test input area and the regex will be evaluated against the entire input. By default, the ^ and $ anchors match the start and end of the whole string. Enable the m (multiline) flag to make ^ and $ match the start and end of each individual line instead. The g (global) flag is required to find all matches throughout the input. Regex is especially powerful when combined with tools like the URL Encoder for parsing encoded query parameters, or the JSON Validator when validating string patterns within JSON data.

Different programming languages implement their own regex engines with varying syntax and feature support. Python, Java, Perl, and .NET all have differences from JavaScript's RegExp engine. If a pattern works in Python but not here, check whether it uses a feature not supported by JavaScript. For testing regex against code strings specifically, the Code Formatter can help you prepare the input: formatting minified code first makes it easier to write accurate line-by-line patterns.