Regular Expressions: A Practical Cheat Sheet

Regular expressions (regex) are powerful pattern-matching tools used in nearly every programming language. This cheat sheet covers the most useful patterns you will encounter in day-to-day development.

Basic Syntax

.       Any character (except newline)
\d     Digit (0-9)
\w     Word character (a-z, A-Z, 0-9, _)
\s     Whitespace (space, tab, newline)
\b     Word boundary
^       Start of string
$       End of string

Quantifiers

*       0 or more
+       1 or more
?       0 or 1 (optional)
{n}     Exactly n times
{n,m}   Between n and m times
{n,}    n or more times

Common Patterns

Email:     [\w.-]+@[\w.-]+\.\w+
URL:       https?://[^\s]+
Phone:     \d{3}[-.]?\d{3}[-.]?\d{4}
IP v4:     \d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}
Date:      \d{4}-\d{2}-\d{2}
Hex color: #[0-9a-fA-F]{6}
HTML tag:  <([a-z]+)[^>]*>.*?</\1>

Character Classes

[abc]       a, b, or c
[^abc]      Not a, b, or c
[a-z]       a through z
[A-Za-z]    Any letter
[0-9]       Any digit

Groups and Capturing

(abc)       Capture group
(?:abc)     Non-capturing group
(?=abc)     Positive lookahead
(?!abc)     Negative lookahead
a|b         a or b (alternation)

Flags

g   Global (find all matches)
i   Case-insensitive
m   Multiline (^ and $ match line boundaries)
s   Dotall (. matches newline)

Test Your Regex

The best way to learn regex is to experiment. Use the Regex Tester to test patterns with live match highlighting. Enter a pattern and test string to see matches in real time.

Tips for Writing Better Regex

Start simple and build up complexity gradually
Use non-capturing groups (?:...) when you do not need to extract the match
Be specific: use \\d instead of [0-9] and \\w instead of [A-Za-z0-9_]
Use anchors (^ and $) when you need to match the entire string
Test with edge cases: empty strings, special characters, very long inputs

Regex has a steep learning curve but pays off enormously. Bookmark this cheat sheet and practice with the tester tool to build your pattern-matching skills.