正規表現テスター

Test patterns against sample text

Regex Tester evaluates a JavaScript regular expression against a string and shows every match, capture group, and start index in real time. It is useful for drafting validation patterns, debugging a search-and-replace, and understanding how flags such as global or multiline change a match.

1. Enter pattern and flags

• Pattern field. Type the body of the regex without surrounding slashes. Backslashes work as in JavaScript source code, so escape literal dots and backslashes.
• Flag field. Single letters such as g, i, m, s. The buttons below toggle the common ones, but you can also type uncommon flags like u or y by hand.
• Presets. Loads a starter pattern for emails, URLs, phone numbers, or ISO dates. They are reasonable defaults rather than perfect validators and can be edited after loading.

2. Run against test text

• Test String area. Paste any text. The tool runs the pattern after every keystroke; large strings are still fast because work happens in your browser.
• Highlighted Matches. Each successful match is wrapped in a highlight so you can see overlap, context, and missed segments.
• Match list. Shows the matched substring, its index, and capture group values. With the global flag every occurrence appears; without it only the first match is shown.
• Copy pattern. Copies the regex in slash-delimited form, ready to paste into source code such as /pattern/flags.

Limits and behavior

The engine is the browser's native RegExp implementation, so features outside ECMAScript (recursion, possessive quantifiers, conditional groups) are not supported. Zero-width matches advance the cursor automatically to avoid infinite loops. Invalid patterns surface the engine's own error message in the alert bar.