What's the difference between * and + in regex?

* matches zero or more occurrences, while + requires one or more. For example, ab*c matches 'ac' and 'abc', but ab+c only matches 'abc' — not 'ac'.

How do I match a literal dot or special character?

Escape it with a backslash: \. matches a literal dot. Other characters needing escaping: ^ $ . * + ? ( ) [ ] { } | \

What are capture groups?

Capture groups, defined with parentheses (), extract matched substrings. Use them to extract parts of a match, backreference within the pattern, or apply quantifiers to a group.

Why is my regex slow or timing out?

Catastrophic backtracking with nested quantifiers like (a+)+ tries exponentially many paths. Fix by restructuring the pattern or using atomic groups.

Regex Tester

Test regular expressions with real-time match highlighting and capture groups.

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What are Regular Expressions?

Regular expressions (regex) are patterns used to match and manipulate text. They're supported in virtually every programming language and text editor. A regex defines a search pattern using special characters — for example, \d+ matches one or more digits, and [a-z]+@[a-z]+\.com matches simple email addresses. Regex is essential for input validation, text parsing, search-and-replace, and log analysis.

Regex Syntax Quick Reference

. — Matches any character (except newline)
\d — Digit (0-9), \D — Non-digit
\w — Word character (a-z, A-Z, 0-9, _), \W — Non-word
\s — Whitespace, \S — Non-whitespace
^ — Start of string, $ — End of string
* — Zero or more, + — One or more, ? — Zero or one
{n,m} — Between n and m occurrences
[abc] — Character class, [^abc] — Negated class
(group) — Capturing group, (?:group) — Non-capturing group
a|b — Alternation (a or b)

Common Regex Patterns

Here are some of the most frequently used regex patterns for common validation and extraction tasks.

// Email (simplified)
^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$

// URL
https?:\/\/[^\s/$.?#].[^\s]*

// IPv4 Address
\b\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}\b

// Date (YYYY-MM-DD)
\d{4}-(?:0[1-9]|1[0-2])-(?:0[1-9]|[12]\d|3[01])

// Phone (US)
\(?\d{3}\)?[-.\s]?\d{3}[-.\s]?\d{4}

// Strong Password (8+ chars, upper, lower, digit, special)
^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)(?=.*[@$!%*?&])[A-Za-z\d@$!%*?&]{8,}$

Regex Flags Explained

g (Global): Find all matches, not just the first
i (Case-insensitive): Match regardless of letter case
m (Multiline): ^ and $ match line boundaries, not just string start/end
s (Dotall): Makes . match newline characters too
u (Unicode): Enables full Unicode matching
y (Sticky): Matches only from the lastIndex position

Frequently Asked Questions

What's the difference between * and + in regex?: * matches zero or more occurrences (the preceding element is optional), while + requires one or more occurrences. For example, ab*c matches 'ac', 'abc', and 'abbc', but ab+c only matches 'abc' and 'abbc' — not 'ac'.
How do I match a literal dot or other special character?: Escape it with a backslash: \. matches a literal dot. Other characters that need escaping include: ^ $ . * + ? ( ) [ ] { } | \. Inside a character class [...], most special characters lose their meaning except ] \ ^ and -.
What are capture groups and when should I use them?: Capture groups, defined with parentheses (), extract matched substrings. Use them when you need to: extract parts of a match (e.g., area code from a phone number), backreference within the pattern (\1), or apply quantifiers to a group. Use (?:...) for grouping without capturing.
Why is my regex slow or causing a timeout?: Catastrophic backtracking occurs with nested quantifiers like (a+)+ or (a|b)*c on non-matching input. The regex engine tries exponentially many paths. Fix by using atomic groups, possessive quantifiers, or restructuring the pattern. Avoid patterns like .*.*.

Guides

Regex for Email Validation Regex for Phone Number Validation Regex for URL Validation Regex for IP Address Validation Regex for Date Format Validation Regex for Password Strength Validation

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