Tester for JavaScript regex: paste a pattern (with gimsuy flags) and a test string, get match highlight, capture groups, match count. Categorized pattern library with ready-made regexes for: email, URL, IPv4/IPv6, MAC address, phone, ISO date, slug, hex color, plus Italian-specific (CF, P.IVA, IBAN). Fully.
Pattern: without delimiters (no /.../), just the body. Flags: g (global, all matches), i (case-insensitive), m (multiline, ^$ match line start/end), s (dotAll. matches newline too), u (unicode), y (sticky).
Free text, multi-line OK. The tester uses the browser's native RegExp engine (V8 / SpiderMonkey / JavaScriptCore), so syntax and perf match standard JS.
Output: original string with matches highlighted in yellow, statistics (total matches, capture groups), match list with index. For capture groups: each match also shows captured groups with their index.
'Pattern library' button opens a categorized list of ready patterns: RFC-friendly email, HTTP/HTTPS URL, IPv4/IPv6, MAC address, Italian phone (landline and mobile), Italian fiscal code, VAT number, IBAN, ISO 8601 date, URL-safe slug. Click to load them in the pattern field.
Differences across regex dialects. JavaScript regexes are ECMAScript-compliant but differ from PCRE (PHP, Perl), POSIX (grep, sed).NET, Python re. Example: variable-length lookbehinds are JS-supported since 2018 (ECMAScript 2018), missing in old browsers. Atomic groups (?>...) missing in JS (present in PCRE/Perl). Recursive patterns missing in JS. For reliable regex testing on complex patterns, use a tester from the same family as the destination runtime.
What this tool offers. Tester with browser-native RegExp, so it represents exactly what happens in production JS code. Match highlight in yellow for visualization. Numbered and named capture groups (syntax (?<name>...)). Match index. Pattern library of 12+ common regexes (Italian-specific included) ready to use.
Performance caveat. The tool does not detect catastrophic backtracking. Patterns like (a+)+ on long input freeze the browser tab. If the tester appears frozen: close the tab. For backtracking audit: redos-checker or dedicated tools. In production, use RE2 (linear-time, no backtracking) for regexes on untrusted input.
| Category | Pattern | Example |
|---|---|---|
^[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\.[A-Za-z]{2,}$ | [email protected] | |
| HTTP/HTTPS URL | ^https?://[^\s]+$ | https://www.example.com |
| IPv4 | ^(?:\d{1,3}\.){3}\d{1,3}$ | 192.168.1.1 |
| MAC address | ^([0-9A-Fa-f]{2}[:-]){5}[0-9A-Fa-f]{2}$ | aa:bb:cc:dd:ee:ff |
| Italian landline phone | ^(?:\+39)?0\d{1,4}\s?\d{4,8}$ | +39 06 1234567 |
| Italian mobile phone | ^(?:\+39)?3\d{2}\s?\d{6,7}$ | +39 333 1234567 |
| Italian fiscal code | ^[A-Z]{6}\d{2}[A-Z]\d{2}[A-Z]\d{3}[A-Z]$ | RSSMRA80A01H501U |
| Italian VAT | ^\d{11}$ | 12345678901 |
| Italian IBAN | ^IT\d{2}[A-Z]\d{10}[A-Z0-9]{12}$ | IT60X0542811101000000123456 |
| ISO 8601 date | ^\d{4}-\d{2}-\d{2}$ | 2026-04-25 |
| URL-safe slug | ^[a-z0-9]+(?:-[a-z0-9]+)*$ | my-tool-name |
| Hex color | ^#(?:[0-9a-fA-F]{3}){1,2}$ | #abc, #aabbcc |
Technical terms used on this page, briefly explained.
(...) that captures a substring. Numbered from 1 (group 0 = full match). Named group: (?<name>...). Non-capturing: (?:...). (?=...) positive lookahead (match followed by). (?!...) negative lookahead. (?<=...) lookbehind. Variable-length supported in JS since 2018. (a+)+ on input aaaaab have exponential complexity (catastrophic backtracking). lastIndex position (no advancing). Useful for tokenizing parsers. Different from g which can skip non-matchable characters. /pattern/flags or constructor new RegExp('pattern', 'flags'). Literal is compiled at parse-time (no runtime overhead). Constructor is useful for dynamic patterns (e.g. user input). Caveat: in the constructor, escape sequences must be doubled ('\\d' for \d).(a+)+, (.*)*, alternations with common prefix. Three strategies: (1) refactor to a linear pattern (e.g. a+ instead of (a+)+); (2) atomic groups or possessive quantifiers (NOT available in JS, only PCRE); (3) use RE2 server-side for untrusted input (Node.js: re2 npm package; Go: regexp standard library).(?P<name>...) Python does NOT work in JS (different Python syntax). JS uses (?<name>...) ES2018+.^[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\.[A-Za-z]{2,}$ covers 99.99% of real cases. For rigorous validation: send confirmation email + click link.m enables ^ and $ to match at start/end of each line (not just full string). Without m, ^ matches only the very start of input. Example: ^abc with m matches 'abc' at every line start.u (Unicode) flag enabled. Syntax: \p{Letter} matches any Unicode letter (including accents, Cyrillic, etc). \P{Letter} NON-letter. Supported in all modern browsers (2020+).Maurizio Fonte, senior IT consultant with 20+ years in PHP, Laravel, unmanaged Linux infrastructure, applied cybersecurity and AI/LLM integration. Production backends, legacy code modernization, security audits, custom AI agents and MCP servers: the work behind every tool published here.
About Maurizio Fonte