Developer
Regular Expressions — A Practical Guide for Developers
Learn regular expressions from scratch. Syntax, common patterns, flags, and practical examples with our free regex tester.
Regular expressions (regex) are one of the most powerful tools in a developer's toolkit. They let you search, match, and manipulate text with precision.
What Is a Regular Expression?
A regular expression is a pattern that describes a set of strings. You use it to:
- Search for specific text patterns
- Validate input (emails, phone numbers, etc.)
- Replace text programmatically
- Extract data from unstructured text
Basic Syntax
Literal Characters
/hello/ matches "hello"
/123/ matches "123"Special Characters
| Pattern | Matches | Example |
|---|---|---|
| . | Any character (except newline) | h.t → "hat", "hot" |
| \d | Any digit (0-9) | \d{3} → "123" |
| \w | Word character (a-z, A-Z, 0-9, _) | \w+ → "hello_world" |
| \s | Whitespace | \s+ → " " |
| ^ | Start of string | ^hello |
| $ | End of string | world$ |
| * | 0 or more | a* → "", "a", "aaa" |
| + | 1 or more | a+ → "a", "aaa" |
| ? | 0 or 1 | colou?r → "color", "colour" |
| {n} | Exactly n times | \d{4} → "2025" |
| {n,m} | Between n and m times | \d{2,4} → "12", "1234" |
Common Patterns
Email Validation
URL Matching
Phone Number (US)
IP Address
Regex Flags
| Flag | Meaning | Example |
|---|---|---|
| g | Global (all matches) | /hello/g |
| i | Ignore case | /hello/i |
| m | Multiline | /^hello/m |
| s | Dot matches newline | /./s |
| u | Unicode support | /\p{Emoji}/u |
Groups and Capturing
Use parentheses to capture parts of a match:
/(\d{4})-(\d{2})-(\d{2})/
// Matches "2025-05-10", captures year, month, dayWhen NOT to Use Regex
- Parsing HTML/XML — Use a proper parser instead
- Complex nested structures — Regex can't handle recursion well
- Very large files — Performance issues with massive text
Test Your Regex
Use our free regex tester to test patterns in real-time with highlighting, flags, and match details.