Question 1

What regex flavor does this tool use?

Accepted Answer

It uses JavaScript's built-in RegExp engine, which follows the ECMAScript specification. This is the same engine used in all major browsers and Node.js. Most common regex patterns work identically across languages, but some features like lookbehinds or named groups may differ from PCRE or Python.

Question 2

What do the flags g, i, and m do?

Accepted Answer

The g (global) flag finds all matches instead of stopping at the first one. The i (case-insensitive) flag makes the pattern match uppercase and lowercase letters equally. The m (multiline) flag changes ^ and $ to match the start and end of each line rather than the entire string.

Question 3

How do I see capture groups?

Accepted Answer

Add parentheses to your pattern to create capture groups, for example (\w+)@(\w+). The results panel shows each match alongside its numbered capture groups. Named groups using the (?<name>...) syntax are also supported and shown by name.

Question 4

Why does my pattern show an error?

Accepted Answer

If the pattern is syntactically invalid, the input turns red and the error message is shown. Common causes are unbalanced parentheses, invalid escape sequences, or unsupported syntax. The tool uses try-catch around RegExp construction so invalid patterns never crash the page.

Question 5

Is there a length limit on the test string?

Accepted Answer

There is no hard limit enforced by the tool, but very large strings with global patterns that produce thousands of matches may slow down the browser. For most testing scenarios, a few paragraphs of text is more than enough.

Regex Tester

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