What is a regular expression?
A regular expression (regex) is a pattern that describes a set of strings — used to search, validate, or extract text. Instead of matching one exact string, a regex can match "three digits, a dash, four digits" or "anything that looks like an email address" in a single compact pattern.
Common flags
g— global, find all matches instead of stopping at the firsti— case-insensitive matchingm— multiline,^/$match line boundaries, not just string boundariess— dotAll, lets.match newlines too
Frequently asked questions
What regex engine does this use?
Your browser's native JavaScript RegExp engine — the same one your code runs against if you're testing patterns for a JS or Node.js project. Other languages (Python, PCRE, Java) have small syntax differences.
Why do I see capture groups listed below the matches?
Parentheses in a pattern like (\d+)-(\d+) create capture groups — parts of each match you can extract individually. This tool lists each match's captured groups so you can see exactly what a group like $1 would contain.
Is my text uploaded when I use this tool?
No. Matching happens instantly in your browser using JavaScript's built-in regex engine — nothing is sent to a server.