What is the difference between .* and .+?

.* matches zero or more of any character (including an empty string). .+ requires at least one character. Use .+ when the match cannot be empty.

How do I make a regex case-insensitive?

Add the i flag. In JavaScript: /pattern/i. In Python: re.compile('pattern', re.IGNORECASE). In grep: use -i flag.

What is a capturing group vs a non-capturing group?

A capturing group (abc) stores the matched text for later reference (e.g. $1 in a replacement). A non-capturing group (?:abc) groups for quantifiers or alternation without storing the match, which is faster.

How do I match a literal backslash?

Use \\ in your regex pattern. Since backslash is the escape character, you need two backslashes: the first escapes the second, resulting in a literal \.

Formula Guide

How to Write and Test Regular Expressions

A regular expression (regex) is a pattern that matches text. It is used to validate input (email, phone, password), search and replace text, extract data from logs, and parse structured content. The same regex syntax works across Python, JavaScript, Java, and most other languages with minor differences. Once you understand the core building blocks, you can construct complex patterns from simple parts.

Last updated: March 31, 2026

The Formula

Anchors:      ^ (start), $ (end)
Character classes: [abc] matches a, b, or c | [a-z] range | \d digit | \w word char | \s whitespace
Quantifiers:  * (0+), + (1+), ? (0 or 1), {n} exactly n, {n,m} between n and m
Groups:       (abc) capturing group | (?:abc) non-capturing | | alternation
Lookaheads:   (?=...) positive | (?!...) negative

Flags: g = global (find all matches), i = case-insensitive, m = multiline (^ and $ match line boundaries). In JavaScript: /pattern/gi

Variable Definitions

Symbol	Name	Description
.	Wildcard	Matches any single character except newline
\d, \w, \s	Shorthand Classes	\d = [0-9], \w = [a-zA-Z0-9_], \s = [ \t\n\r]
^, $	Anchors	^ matches start of string/line, $ matches end

Step-by-Step Example

Write a regex to validate a basic email address like user@example.com.

Given

Target format:user@domain.tld

Solution

1
Match local part (letters, digits, dots, underscores): [a-zA-Z0-9._%+-]+
2
Match the @ symbol literally: @
3
Match domain name: [a-zA-Z0-9.-]+
4
Match a dot before TLD: \.
5
Match TLD (2-6 letters): [a-zA-Z]{2,6}
6
Add anchors for full-string match: ^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,6}$

Pattern: /^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,6}$/i — matches user@example.com ✓, rejects user@, @domain.com ✗

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Common Mistakes to Avoid

Forgetting to escape the dot (.) — unescaped . matches any character. Use \. when you mean a literal dot.

Using greedy quantifiers when lazy is needed — .* is greedy and matches as much as possible. .*? is lazy and stops at the first match.

Anchoring with ^ and $ but enabling multiline flag unintentionally — in multiline mode, ^ and $ match line boundaries, not the whole string.

Not escaping special characters in strings — in many languages, the backslash in \d needs to be \\d in a string literal.

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