Regex Tester

How to test a regular expression

1. Type your pattern between the slashes and choose flags.
2. Paste test text. Matches are highlighted live and listed with their position and capture groups.
3. Optionally enter a replacement to preview a find-and-replace.

Regex cheat sheet

• . any character · \d digit · \w word character · \s whitespace
• * 0 or more · + 1 or more · ? optional · {2,5} between 2 and 5
• ^ start · $ end · \b word boundary
• [abc] one of · [^abc] none of · a|b either
• (…) capture group · (?:…) non-capturing · (?<name>…) named group
• (?=…) lookahead · (?<=…) lookbehind

Avoid catastrophic backtracking

Nested quantifiers like (a+)+ can take exponential time on certain input and freeze a page or server. Prefer specific character classes over .*, and anchor patterns where possible.

Related: Diff Checker, JSON Formatter.

Understanding flags

• g (global): find all matches instead of stopping after the first.
• i (ignore case): /hello/i also matches “Hello” and “HELLO”.
• m (multiline): ^ and $ match at the start and end of each line, not just the whole text.
• s (dotAll): . also matches line breaks.
• u (unicode): treats emoji and other characters outside the basic plane as one character and enables \p{…} property escapes like \p{L} for any letter.
• y (sticky): only matches starting exactly at the current position.

Greedy vs. lazy

Quantifiers like * and + are greedy: they match as much as possible. In the text <b>a</b><b>c</b>, the pattern <b>.*</b> matches everything from the first tag to the last. Adding a question mark makes them lazy: <b>.*?</b> matches each bold section separately.

Using your pattern in other languages

In JavaScript you can use the pattern directly as /pattern/flags. In Python, use a raw string r"pattern" with re.findall; Python uses (?P<name>…) for named groups. In Java and C#, remember to double every backslash inside string literals. Lookbehind and Unicode property escapes are supported in most modern engines, but not in every older one.

Validate, don’t over-validate

Regular expressions are great for finding and extracting text, but trying to fully validate complex formats like email addresses with one pattern usually rejects valid input. Use a simple pattern to catch obvious mistakes and rely on a confirmation step for the rest.

Testing replacements

The replacement box previews a find-and-replace on your test text. Use it to check reformatting before running it on real data, for example turning dates from 2026-11-12 into 12/11/2026 with the pattern (\d+)-(\d+)-(\d+) and the replacement $3/$2/$1.