Regular expressions (regex) are one of the most powerful — and notoriously cryptic — tools in a developer's toolkit. A single well-crafted regex can replace dozens of lines of string manipulation code.
Test Regex Online
Use the Regex Tester to write and test patterns interactively:
- Matches highlight in real time as you type
- See all match groups and captured values
- Test against multiple lines or a full block of text
- Switch flags (global, case-insensitive, multiline) instantly
Regex Flags
| Flag | Meaning |
|---|---|
g | Global — find all matches, not just the first |
i | Case-insensitive matching |
m | Multiline — ^ and $ match start/end of each line |
s | Dot-all — . matches newlines too |
u | Unicode mode — enables full Unicode support |
Essential Patterns Cheatsheet
Character Classes
\d— digit (0–9)\w— word character (a–z, A–Z, 0–9, _)\s— whitespace (space, tab, newline)[a-z]— character range[^abc]— negated class (not a, b, or c)
Quantifiers
*— 0 or more+— 1 or more?— 0 or 1 (optional){3}— exactly 3{2,5}— between 2 and 5
Anchors & Boundaries
^— start of string (or line withmflag)$— end of string (or line withmflag)\b— word boundary
Common Patterns
- Email:
[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,} - URL:
https?:\/\/[^\s]+ - IPv4:
(\d{1,3}\.){3}\d{1,3} - Hex color:
#([0-9a-fA-F]{3}|[0-9a-fA-F]{6})\b - ISO date:
\d{4}-\d{2}-\d{2}
Lookahead & Lookbehind
(?=...)— positive lookahead: match if followed by(?!...)— negative lookahead: match if NOT followed by(?<=...)— positive lookbehind: match if preceded by(?<!...)— negative lookbehind: match if NOT preceded by
Example — match a number followed by "px": \d+(?=px)
Named Capture Groups
(?<year>\d{4})-(?<month>\d{2})-(?<day>\d{2})Named groups (e.g., match.groups.year) make regex results much easier to work with in code.