Regex tester
Regular expressions pattern-match text for validation, search-replace, and log parsing. Test patterns with live highlighting before dropping into production code — avoid catastrophic backtracking surprises on user input.
Pattern
Flags
Test text
Output
[0] Hello
[6] world
[12] 123
How to test a regex
1. Enter regex pattern (without outer slashes or with flags).
2. Set flags: g global, i case insensitive, m multiline.
3. Paste sample text in test string area.
4. Review matches, capture groups, and replace preview if available.
Regex examples
Email-ish validation
^[\w.-]+@[\w.-]+\.\w+$ catches simple emails — not RFC-perfect but useful for forms.
Extract ISO dates
\d{4}-\d{2}-\d{2} finds 2024-03-15 in log lines for grep replacement.
Slugify helper
Replace [^a-z0-9]+ with hyphen for URL slug from title string.
When regex helps
• When validating structured strings (phone, postal code).
• When parsing semi-structured logs.
• When learning capture groups and lookahead.
When to avoid regex
• When parsing HTML or JSON — use proper parsers.
• When regex becomes unreadable — split into code logic.
• When ReDoS risk on user-supplied patterns in server — timeout and limit length.
ReDoS awareness
Nested quantifiers like (a+)+ on evil string hang CPU — avoid user regex on server or use safe engines with timeouts.
Validate vs extract
Anchored pattern ^...$ validates whole string; unanchored finds substring anywhere.
Replace in editors
VS Code uses similar JS regex in find-replace — test here then paste pattern.
Frequently asked questions
Which regex flavor?
JavaScript RegExp — PCRE differences exist (lookbehind support varies by engine version).
Global flag g?
Finds all matches not just first — testStateful with lastIndex in JS.
Escape special chars?
. * + ? [ ] ( ) { } | \ ^ $ need escaping for literal match.
Capture groups?
Parentheses ( ) create numbered groups accessible in replace $1 $2.
Local?
Yes.
Multiline ^ $?
m flag makes ^ $ match line boundaries not whole string.