What a regular expression is

A regular expression (regex / regexp) is a tiny pattern-matching language. The pattern \d{3}-\d{4} matches 555-1212. The pattern ^https?:\/\/[\w.-]+\.[a-z]{2,}\b matches a URL. Regex is supported in every programming language, every text editor, and most CLI tools (grep, sed, awk) — once you know it, it follows you everywhere.

Regex is also famously easy to get subtly wrong. A pattern that looks right often greedy-matches more than you want, or misses Unicode characters, or doesn't anchor properly. A live tester is the standard way to write regex: paste the input you actually have, type the pattern, see what matches in real time.

What this tool does

• Live matching as you type. Matches are highlighted in the input; capture groups appear in a side panel.
• All standard flags — g (global), i (case-insensitive), m (multiline ^ and $), s (dotall), u (Unicode), y (sticky).
• Named capture groups ((?<name>…)) are extracted and labelled.
• Find-and-replace preview. Type a replacement string with $1, $<name>, etc. and see the output side-by-side.
• Cheatsheet panel with the syntax reference open. No more popping between three tabs.
• JavaScript flavour. The engine is the browser's built-in RegExp, so what you see here matches what your JS code will see at runtime. (PCRE / Python flavour differences are noted in the cheatsheet.)
• Browser-only. Paste sensitive logs, API responses, or credentials — nothing uploads.

Common patterns to copy

• Email (rough): ^[^\s@]+@[^\s@]+\.[^\s@]+$ — good enough for most form validation. Don't try to write RFC 5322-compliant; that pattern is famously ~6000 characters long.
• URL: ^https?:\/\/[\w.-]+\.[a-z]{2,}(\/.*)?$ — http or https, domain, optional path.
• US phone: \(?\d{3}\)?[-.\s]?\d{3}[-.\s]?\d{4} — handles 555-555-5555, (555) 555-5555, 555.555.5555.
• ISO 8601 date: ^\d{4}-(0[1-9]|1[012])-(0[1-9]|[12]\d|3[01])$ — strict YYYY-MM-DD with valid months and days.
• IPv4: ^(\d{1,3}\.){3}\d{1,3}$ (loose) or ^((25[0-5]|2[0-4]\d|[01]?\d?\d)\.){3}(25[0-5]|2[0-4]\d|[01]?\d?\d)$ (strict).
• Hex color: ^#?([a-fA-F0-9]{6}|[a-fA-F0-9]{3})$.
• UUID v4: ^[0-9a-f]{8}-[0-9a-f]{4}-4[0-9a-f]{3}-[89ab][0-9a-f]{3}-[0-9a-f]{12}$.

When NOT to use regex

• Parsing HTML. Use a real HTML parser (DOMParser, cheerio, BeautifulSoup). Regex breaks on every edge case.
• Parsing JSON / YAML / TOML. Use a real parser; the Toolenza json and data-convert tools.
• Anything with nested structures. Regex is regular — it can't count balanced parens.