Regex Tester

A regular expression, often called a regex, is a pattern for searching, matching, validating, and processing text. Developers use regex for tasks such as locating values, checking input formats, and extracting structured parts from strings.

This tester uses JavaScript regular expression syntax. Results can differ from regex engines used by other programming languages and platforms.

1. Enter a pattern without surrounding slash characters.
2. Select the JavaScript flags required by the pattern.
3. Paste or type a test string.
4. Select Test Regex.
5. Review the match summary, highlighted text, indexes, and capture groups.

• g finds successive matches globally.
• i enables case-insensitive matching.
• m changes line anchors so ^ and $ work across lines.
• s enables DotAll behavior so a dot can match line terminators.
• u enables Unicode-aware pattern processing.
• y enables sticky matching at the current lastIndex.

An email-like pattern can locate addresses in ordinary text:

Pattern: \b[\w.%+-]+@[\w.-]+\.[A-Za-z]{2,}\b
Test: alice@sample.test
Match: alice@sample.test

This example demonstrates matching only. Production email validation usually requires application-specific rules beyond a single regular expression.

Parentheses such as (...) create numbered capture groups. A pattern like(\w+)@(\w+\.\w+) can preserve the username as Group 1 and the domain as Group 2.

JavaScript named groups use syntax such as (?<user>...). When the browser provides named group results, this tester lists each name and value separately.

A greedy quantifier such as .* consumes as much text as possible while still allowing the pattern to match. A lazy quantifier such as .*? consumes as little as possible. Choose the form that matches the boundaries your data requires.

• Forgetting to escape a character that has special regex meaning.
• Leaving a group or character class unclosed.
• Selecting the wrong flags or forgetting a required flag.
• Using anchors that do not match the intended line behavior.
• Using an overly broad expression such as .*.

Structured formats should use their own parser where possible. For JSON grammar and parsing errors, use the JSON Validatorinstead of approximating complete JSON syntax with a regular expression.

JavaScript RegExp, PCRE, Python, Java, .NET, and RE2 share many concepts, but their syntax, feature support, flags, and edge cases are not identical. A successful result in this JavaScript regex tester is not a guarantee that the same pattern works unchanged elsewhere.

Some patterns can trigger catastrophic backtracking when applied to particular input. JavaScript RegExp has no simple built-in timeout, so complex or untrusted patterns can temporarily block the browser thread. Keep patterns specific and test them with realistic, bounded input before production use.

• Check a JavaScript pattern against representative input before adding it to code.
• Compare global, case-insensitive, multiline, Unicode, and other flag behavior.
• Inspect capture groups and match indexes while debugging extraction logic.
• Identify greedy matching, escaping mistakes, or patterns that may backtrack heavily.