Regex Tester (Python re Syntax)
Test regular expressions against sample text using Python's re engine, the exact behaviour your Python code will have, including named groups, flags and non-greedy quantifiers. See every match and captured group highlighted as you refine the pattern.
How to use
- Enter your pattern, e.g. (?P<order>\d+).
- Paste the test text you want to match against.
- Toggle flags: IGNORECASE, MULTILINE, DOTALL as needed.
- Run and inspect matches, spans and captured groups. Adjust until correct.
Regular expressions are a pattern language for text, and every engine speaks a slightly different dialect. This tester runs Python's re module specifically, which matters: Python uses (?P<name>...) for named groups where JavaScript uses (?<name>...), has no /flags suffix syntax, applies backtracking rules of its own, and offers \A and \Z anchors alongside ^ and $. A pattern verified against a JavaScript-based tester can behave differently the moment it lands in your Python code, testing in the target dialect removes that whole class of surprise.
The distinctions that bite most often in Python: re.match anchors at the start of the string while re.search scans anywhere, a mismatch responsible for countless 'my regex works elsewhere' bugs. Greedy quantifiers (.*) grab the longest possible match, so extracting quoted text needs the non-greedy .*? or, better, a negated class like [^"]*. The DOTALL flag decides whether . crosses newlines. MULTILINE decides whether ^ and $ match at every line. And always write patterns as raw strings (r"\d+") so Python's own backslash escaping doesn't silently eat your metacharacters.
One performance note worth internalising: nested quantifiers like (a+)+ can trigger catastrophic backtracking, where match time explodes exponentially on non-matching input, a real denial-of-service vector if the pattern runs on user input. Prefer specific character classes over .* and test patterns against inputs that should fail, not only ones that should succeed.
Examples
Input: pattern: \d+ text: order 42 shipped
Output: 1 match: '42' at span (6, 8)Input: pattern: (?P<user>\w+)@(?P<domain>[\w.]+) text: contact [email protected]
Output: match '[email protected]', groups: user='asha', domain='example.com'Input: pattern: ".*?" text: say "a" and "b"
Output: 2 matches: '"a"' and '"b"' (greedy .* would match '"a" and "b"' as one)Frequently asked questions
Why does my regex work in JavaScript but not Python?
The dialects differ: named groups are (?P<name>...) in Python, lookbehind rules differ, and Python has no inline /gi flag syntax, flags are passed separately or written as (?i) at the pattern start. Test in the engine you deploy on.
What's the difference between re.match and re.search?
re.match only succeeds if the pattern matches at position 0 of the string, re.search finds the pattern anywhere. Most 'pattern not found' confusion in Python is using match when search was intended.
Why does .* match more than I wanted?
Quantifiers are greedy by default, they consume as much as possible while still allowing a match. Append ? for the non-greedy version (.*?), or use a negated character class like [^,]* which is both precise and faster.
Should I validate emails with a regex?
Only loosely. The full RFC 5322 grammar is famously not regex-friendly, and a strict pattern rejects valid addresses. Check for a basic [email protected] shape, then verify deliverability with an actual email or an MX check.
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