Question 1

How can I test a regular expression online for free?

Accepted Answer

Type your regex and test text into the Regex Tester. Matches highlight live as you type, capture groups are listed separately, and each token of your pattern is explained — so you can debug and refine patterns instantly, free.

Question 2

Which regex flavour does this tester use?

Accepted Answer

It uses the JavaScript regular expression engine built into your browser, the same one used by Node.js and web apps. Most syntax transfers to other languages, but lookahead, named groups and flag support can differ slightly in engines like PCRE, Python or Java.

Question 3

What do the flags mean?

Accepted Answer

g finds all matches instead of stopping at the first. i makes matching case-insensitive. m lets ^ and $ match at line boundaries. s makes the dot match newline characters too. u enables full Unicode support, and y makes matching sticky, starting exactly at the last position.

Question 4

How do capture groups work?

Accepted Answer

Parentheses in your pattern create capture groups. Each match lists the groups in order, so group 1 is the text matched by the first pair of parentheses. Use (?:...) when you need grouping without capturing, and $1, $2 in the replacement field to insert a group into the output.

Question 5

Why does my regex match nothing?

Accepted Answer

Common causes are unescaped special characters (a literal dot needs \.), a missing g flag when you expect several matches, anchors that do not fit your text, or character classes that exclude the characters you are looking for. Shorten the pattern piece by piece to find the part that stops matching.

Question 6

Can a regex freeze my browser?

Accepted Answer

Patterns with nested quantifiers can cause catastrophic backtracking on long inputs. This tester runs in your browser tab, so keep test strings modest, avoid patterns like (a+)+ on text that almost matches, and reload the tab if a pattern does lock up.

Regex Tester

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How to use the Regex Tester

Write your pattern

Add test text

Review matches and groups

About the Regex Tester

Where it helps most

Live matching

Capture groups

Find and replace

Flag testing

Using it well

Backtracking danger

Mundane failures

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