Regex Tester
FAQ
Everything you need to know about regular expressions
What is a regular expression (regex)?
A regular expression is a sequence of characters that defines a search pattern. It is used to find, replace or validate text strings. For example, the pattern \d+ matches one or more digits, while [a-z]+ matches one or more lowercase letters.
What do the flags g, i and m mean?
The "g" (global) flag finds all matches instead of stopping at the first. The "i" (case-insensitive) flag ignores the difference between upper and lower case. The "m" (multiline) flag makes ^ and $ match the beginning and end of each line.
What are capture groups?
Capture groups are parts of the pattern enclosed in round parentheses ( ). They allow you to extract specific portions of a match. For example, the pattern (\d{4})-(\d{2})-(\d{2}) on "2024-01-15" captures "2024", "01" and "15" as three separate groups.
Why do I get an "Invalid regular expression" error?
This error appears when the pattern contains invalid syntax. Common causes: unclosed parentheses or square brackets, incorrect quantifiers (e.g. * without preceding character), unescaped special characters. Use \\ to escape special characters like . * + ? ^ $ { } [ ] | ( ).
Does my data stay private?
Yes, and not because we promise it: this tool runs entirely inside your browser. What you type or upload never leaves your device, so it never reaches a server of ours and there is nothing we could keep, read or lose. You can check it yourself: open the page, disconnect from the internet, and the tool keeps working.
Do you store what I enter?
No. There is no database behind this tool: we do not store your files, your text or your results. When you close the tab nothing is left — not here with us, because it never arrived, and not anywhere else.
Do you profile me? Are you GDPR compliant?
We do not profile you: there is no account to create, we never ask for your email to use a tool, and we keep no profile of what you do. We are an Italian company and we work under the GDPR and the other European rules, with the privacy notice and the consent request in plain sight. One thing we say in full rather than hiding it behind a slogan: the ads on the page are Google's, and Google may use cookies to personalise them. You decide from the consent banner: if you say no the ads stay, but they are not personalised, and the tool works exactly the same.
If it is free, how do you make money?
From the ads on the page, and that is all. We do not sell your data, there is no hidden paid plan, there is no limit that eventually asks for your card, and you do not have to sign up. The ads pay for the servers and for the time we spend writing the tools: that is how we can keep them open to everyone.