About this tool
regexcheck.com tells you whether a pattern matches; regexnow is for doing something with the matches. Paste a log, a CSV column, an export — anything — and pull out every email or ID (extract), keep only the lines that contain ERROR or drop the noise (filter), or rewrite with capture groups like $1-$2 (replace).
Patterns run in a Web Worker with a 2-second kill switch, so a catastrophically backtracking regex can never freeze the tab, and nothing you paste leaves your browser. Need to just test a pattern first? Use the sister tool regexcheck.com.
The engine is JavaScript’s own — the same flavor you use in Node and the browser — so \d, \w, lookaheads, named groups (?<name>…) and the i, m, s flags behave exactly as they do in your code. In extract mode, capture groups are output tab-separated, which pastes cleanly into a spreadsheet. Filter mode works like grep (inverted, like grep -v), and replace supports $1, $& and $<name> substitutions. Common jobs: pull emails, URLs or IPs from a log, strip timestamps, reorder date parts.
Frequently asked questions
What regex flavor does this tool use?
JavaScript (ECMAScript) regex — the same engine as Node.js and every browser. Lookaheads, lookbehinds, named groups and unicode escapes all work; PCRE-only features such as atomic groups or possessive quantifiers are not supported.
How do I extract only part of each match?
Add a capture group with parentheses. In extract mode every output line shows the full match plus each group, separated by tabs — so (\w+)@([\w.]+) gives the address, user and domain as columns that paste straight into a spreadsheet.
Why was my pattern stopped after 2 seconds?
It almost certainly hit catastrophic backtracking — nested quantifiers like (a+)+ can take exponential time. Every pattern runs in a Web Worker with a 2-second kill switch so the tab never freezes; rewrite with lazy quantifiers or more specific character classes.
Is my text uploaded anywhere?
No. Matching, filtering and replacing all run inside a Web Worker in your browser. Logs, exports or customer data you paste never leave your machine — safe to use with sensitive material.