The regex tester builds your expression with the browser's JavaScript RegExp directly and supports the six togglable flags g i m s u y. The result area lists match positions, text and numbered or named capture groups, and a replacement box shows the real result of JavaScript String.replace().

1. 01Enter a pattern without the surrounding / and tick the flags you need.
2. 02Enter the test text and read the match highlighting, positions and capture groups.
3. 03Enter a replacement; $1, $<name> and the rest follow JavaScript replacement semantics.
4. 04Click a built-in example to fill in the pattern, flags and sample text together.

• The syntax is exactly the current browser's JavaScript RegExp — not PCRE, RE2, Python or Java; unsupported syntax reports a construction error straight away.
• Internally the page tries to add the d flag to obtain capture group indices and falls back automatically on older browsers; this does not change the flags you see.
• To enumerate matches it ensures a g internally, but if you did not tick g the display still keeps only the first match; the replacement strictly uses your original flags, so without g only the first occurrence is replaced.
• A zero-length match has its lastIndex advanced manually to avoid an infinite loop; at most 1000 matches are collected.
• Test text longer than 50,000 characters disables whole-text highlighting to keep rendering cheap; the match analysis still runs, and the list itself shows only a limited number of entries.

The pattern, the test text and the replacement are executed locally in the browser only; nothing is uploaded and nothing is saved.