Question 1

What is a regular expression?

Accepted Answer

A search pattern. Used for finding text, validating input, extracting data, find & replace. Supported in most languages, editors, and CLI tools. Examples: validate emails, extract phone numbers, search logs.

Question 2

How do I test if my regex pattern works?

Accepted Answer

Type pattern. Paste test text. Matches highlight in real-time. See match count, capture groups, positions. Test with multiple strings to make sure it works for all cases.

Question 3

What are regex flags and which ones should I use?

Accepted Answer

g (global): find all matches, not just first. i (case insensitive): ignore case. m (multiline): ^ and $ match line breaks. s (dotAll): . matches newlines. u (unicode): unicode mode. y (sticky): match from last index. Most common: g and i.

Question 4

How can I learn regular expressions?

Accepted Answer

Start with pattern library (common patterns). Use pattern explainer (breaks down what each part does). Practice simple patterns first (literal text, character classes). Then move to advanced stuff (lookarounds, backreferences). Cheat sheet for quick reference.

Question 5

Can I generate code from my regex pattern?

Accepted Answer

Yep. JavaScript, Python, Java, PHP, Ruby, Go, C#, Rust, SQL, Perl. Shows matching and replacement examples. Proper syntax, imports, escaping. Copy-paste ready.

Question 6

What is catastrophic backtracking and how do I avoid it?

Accepted Answer

Regex engine tries exponential possibilities and freezes. Happens with nested quantifiers like (a+)+ or overlapping patterns. Avoid: use specific patterns, avoid nested quantifiers, use possessive quantifiers. Tool warns you about problematic patterns.

Question 7

How do capture groups work?

Accepted Answer

Parentheses () capture parts of a match. (\d{3})-(\d{3})-(\d{4}) on '555-123-4567' captures group 1: '555', group 2: '123', group 3: '4567'. Named groups: (?<area>\d{3}). Non-capturing groups: (?:...) when you need grouping without capture.

Question 8

What's the difference between greedy and lazy matching?

Accepted Answer

Greedy (*, +, {n,}): match as much as possible. Lazy (*?, +?, {n,}?): match as little as possible. In 'abc123def456', \w+\d+ (greedy) matches all. \w+?\d+ (lazy) matches just 'abc123'. Use lazy to stop at first match, not last.

Question 9

How do I match special characters literally?

Accepted Answer

Escape with backslash. . * + ? ^ $ { } [ ] ( ) | \ are special. To match a literal dot: \. (not just .). To match backslash: \. Pattern library shows proper escaping for URLs, paths, emails.

RegEx Tester & Builder

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