Question 1

What is a regular expression (regex) and when should I use it?

Accepted Answer

Regular expression (regex) is a pattern-matching language for searching and manipulating text. Uses special characters like . (any character), * (zero or more), + (one or more), ? (optional), [] (character class), () (group). Common uses: validate email/phone formats, extract data from logs, find/replace in code editors, parse URLs, sanitize user input, search documents, split strings. Example: /\d{3}-\d{2}-\d{4}/ matches SSN format 123-45-6789. More powerful than simple string search but harder to read. Use for complex patterns; use indexOf() for simple exact matches. This tool tests patterns with live highlighting.

Question 2

What are the most common regex patterns I should know?

Accepted Answer

Email: /^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$/. URL: /https?://(www\.)?[-a-zA-Z0-9@:%._\+~#=]{1,256}\.[a-zA-Z0-9()]{1,6}\b/. Phone (US): /^$?\d{3}$?[-.\s]?\d{3}[-.\s]?\d{4}$/. Date (YYYY-MM-DD): /^\d{4}-\d{2}-\d{2}$/. IP address: /^(\d{1,3}\.){3}\d{1,3}$/. Hex color: /^#?([0-9A-Fa-f]{6}|[0-9A-Fa-f]{3})$/. Username (alphanumeric): /^[a-zA-Z0-9_]{3,16}$/. Password (min 8, 1 upper, 1 lower, 1 digit): /^(?=.*[a-z])(?=.*[A-Z])(?=.*\d).{8,}$/. This tool includes common patterns library for quick testing.

Question 3

What are regex flags and when should I use them?

Accepted Answer

Flags modify regex behavior, added after closing delimiter (/pattern/flags). Common flags: g (global) - find all matches, not just first. i (ignore case) - case-insensitive matching (A = a). m (multiline) - ^ and $ match line breaks, not just string start/end. s (dotall) - . matches newlines. u (unicode) - proper unicode character handling. y (sticky) - match at exact position. Example: /test/gi finds all "test", "TEST", "Test" occurrences. Without g, stops after first match. Use g for find-all, i for case-insensitive, m for multi-line text. This tool supports all major flags with real-time preview.

Question 4

How do I use capture groups and backreferences in regex?

Accepted Answer

Parentheses () create capture groups to extract matched portions. Example: /(\d{3})-(\d{3})-(\d{4})/ in "555-123-4567" captures "555", "123", "4567" as groups 1, 2, 3. Backreferences reuse captured groups: \1, \2 refer to first, second group. Example: /(\w+)\s\1/ matches repeated words ("the the"). Named groups: /(?\d{3})-(?\d{3})/ creates named captures. Non-capturing groups: /(?:abc)+/ groups without capturing (faster). Use cases: swap date formats, find duplicate words, extract structured data. JavaScript: match()[1], replace("$1-$2"). This tool highlights capture groups and shows extracted values.

Question 5

What is the difference between greedy and lazy quantifiers?

Accepted Answer

Quantifiers (* + {n,m}) are greedy by default - match as much as possible. Example: /<.*>/ on "text" matches entire string (greedy). Lazy (non-greedy) quantifiers add ? after quantifier - match as little as possible. Example: /<.*?>/ matches "", then "" separately. Greedy: .* (zero or more, greedy), .+ (one or more, greedy), .{2,5} (2-5, greedy). Lazy: .*? .+? .{2,5}?. Use lazy for: HTML tag extraction, avoiding over-matching, parsing nested structures. Use greedy for: full line matching, whole word extraction. This tool shows greedy vs lazy behavior with match highlighting.

Question 6

How do I validate email addresses with regex?

Accepted Answer

Basic email regex: /^[^\s@]+@[^\s@]+\.[^\s@]+$/ (allows most valid emails). Comprehensive: /^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$/ (stricter). Validation challenges: RFC 5322 spec is complex (allows quoted strings, comments), international domains (unicode characters), new TLDs (.museum, .technology), plus addressing (user+tag@domain.com). Best practice: use simple regex for UI validation, send confirmation email for real verification. Don't over-restrict (user@domain.co is valid). This tool tests email patterns against real examples and highlights match groups for debugging custom patterns.

Question 7

What are lookaheads and lookbehinds (regex assertions)?

Accepted Answer

Assertions match position, not characters. Positive lookahead (?=pattern) - matches if followed by pattern. Example: /\d(?= dollars)/ matches "5" in "5 dollars" but not "5 euros". Negative lookahead (?!pattern) - matches if NOT followed by pattern. Positive lookbehind (?<=pattern) - matches if preceded by pattern. Example: /(?<=\$)\d+/ matches "100" in "$100". Negative lookbehind (?<!pattern). Use cases: password validation (must contain uppercase: (?=.*[A-Z])), extract values after labels, match words except in quotes. Browser support: lookbehind added in ES2018. This tool tests all assertion types with position highlighting.

Question 8

How do I debug regex that isn't matching what I expect?

Accepted Answer

Common issues: escaping special characters (. * + ? [ ] ( ) { } ^ $ | \) - use backslash \. Forgetting anchors - /test/ matches "testing" (use ^test$ for exact match). Wrong flags - case sensitivity (add i flag). Greedy vs lazy - .* vs .*?. Character class mistakes - [a-Z] is wrong (use [a-zA-Z]). Debugging steps: start simple, add complexity incrementally. Test with online tool (this tool!). Use verbose mode if available. Check documentation for your regex flavor (JavaScript vs Python vs PCRE differ). Test edge cases. Use visualization tools. This tool provides live highlighting, match explanation, and error messages for syntax issues.

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