validator.regex.noResults
Regex is the only programming tool where writing it and reading it feel like completely different skills.
You can spend twenty minutes crafting a pattern that perfectly matches everything it should — then discover six months later that someone else on your team can't tell what it does. Or that it works perfectly in Python but fails silently in JavaScript because the two engines handle it differently.
Paste your pattern and test strings above. See matches highlighted in real time. See every captured group. See exactly where the pattern succeeds and where it breaks — before it breaks in production.
Most programming constructs have a learning curve that levels off. Regex never fully levels off — even experts reach for a tester for anything beyond trivial patterns.
Patterns like ^(?=.*[A-Z])(?=.*\d).{8,}$ pack enormous meaning into compact syntax. They are nearly unreadable at first glance.
JavaScript regex, Python `re`, Java `Pattern`, and PCRE all share core syntax but diverge on lookbehinds, named groups, and unicode.
Certain patterns fail slowly. ReDoS (Regular Expression Denial of Service) can turn a harmless pattern into a server-crashing vulnerability.
Empty strings. Unicode characters. Multi-line input. The inputs you didn't test are always the ones that reveal the holes.
Matches are highlighted instantly as you type. No submit button. See immediately what matches and what doesn't.
See exactly what `$1`, `$2`, and named groups captured. Critical for data extraction patterns.
Test against the actual engine you use: JavaScript, Python, PCRE (PHP), Java, or Go.
Toggle flags interactively: global (g), case-insensitive (i), multiline (m), dotAll (s).
See total match count and exact start/end positions for every match found.
Catches syntax errors immediately — unclosed groups, invalid escapes — with clear explanations.
| Feature | JavaScript | Python | PCRE (PHP) |
|---|---|---|---|
| Named Groups | (?<name>...) | (?P<name>...) | (?<name>...) |
| Lookbehind | Fixed-width only (ES2018+) | Variable-width supported | Variable-width supported |
| Unicode \w | No (unless /u flag) | Yes (default in Py3) | Yes (with /u modifier) |
ReDoS (Regular Expression Denial of Service) occurs when a regex engine encounters a pathological input that triggers exponential backtracking.
Against the input "aaaaaaaaaaaaaaaaab", this pattern causes many engines to backtrack millions of times, freezing the thread or process.
How to protect: Avoid nested quantifiers. Use atomic groups `(?>...)` if supported. Test against adversarial inputs.
^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$^https?:\/\/(www\.)?[-a-zA-Z0-9@:%._+~#=]{1,256}\.[a-zA-Z0-9()]{1,6}\b([-a-zA-Z0-9()@:%_+.~#?&/=]*)$^((25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.){3}(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)$^[0-9a-f]{8}-[0-9a-f]{4}-4[0-9a-f]{3}-[89ab][0-9a-f]{3}-[0-9a-f]{12}$^\d{4}-(0[1-9]|1[0-2])-(0[1-9]|[12]\d|3[01])$^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)(?=.*[@$!%*?&])[A-Za-z\d@$!%*?&]{8,}$Validator checks syntax. Tester checks matches against strings. Our tool does both interactively.
Differing engine rules. Python supports variable-width lookbehind and `(?P<name>)` groups; JS has fixed-width lookbehind and `(?<name>)` groups.
Global match. Finds ALL matches in the string, not just the first one. Essential for finding multiple occurrences.
When a regex engine takes exponential time to fail a match. Caused by nested quantifiers like `(a+)+`.
Groups part of a pattern (for quantifiers/alternation) without saving the matched text into a numbered capture group.
A word boundary position. It matches the spot between a word char `\w` and a non-word char `\W`. Zero-width.
No. HTML is not a regular language. Use a DOM parser (BeautifulSoup, DOMParser) instead.
`*` is greedy (eats as much as possible). `*?` is lazy (eats as little as possible). `<b>.*</b>` matches whole string; `<b>.*?</b>` matches first tag.
Paste your pattern above, throw every edge case at it, and know exactly what it matches — and what it doesn't.