Regex Tester & Debugger
Test, debug and visualize regular expressions in real time
Regex flags explained
The g flag finds all matches (not just the first). The i flag makes matching case-insensitive. The m flag treats ^ and $ as line anchors. The s flag makes the dot (.) match newlines too.
Capture groups & presets
Parentheses () create capture groups. Use preset patterns for common tasks like validating emails, URLs, phone numbers, IP addresses and ISO dates — then customize them for your use case.
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Frequently asked questions about Regular Expressions
Everything about regex flags, syntax, and common patterns
The g (global) flag finds all matches instead of stopping at the first. The i (ignore case) flag makes the match case-insensitive. The m (multiline) flag makes ^ match the start of each line and $ match the end of each line. The s (dotAll) flag makes the dot (.) match any character including newlines.
The + quantifier matches one or more of the preceding element — it requires at least one match. The * quantifier matches zero or more — it also matches if the element is absent. Use + when the element must appear at least once, and * when it is optional. Both are greedy by default: add ? (like +? or *?) for non-greedy matching.
Lookahead (?=...) matches a position followed by a certain pattern without including it in the match. Negative lookahead (?!...) matches a position NOT followed by a pattern. Lookbehind (?<=...) matches a position preceded by a pattern. These are zero-width assertions — they check context without consuming characters.
Common patterns include: email validation ([a-zA-Z0-9._%+\-]+@[a-zA-Z0-9.\-]+\.[a-zA-Z]{2,}), URL matching (https?:\/\/[^\s]+), IP addresses (\b(?:\d{1,3}\.){3}\d{1,3}\b), and ISO dates (\d{4}-\d{2}-\d{2}). The presets in this tool provide ready-to-use versions of these patterns.