Q: What are regex modifiers/flags?

Regex modifiers change matching behavior: i makes case-insensitive (abc matches ABC), g finds all matches not just first, m makes ^ and $ match line boundaries not just string boundaries, s makes . match newline too, x allows whitespace and comments in pattern. JavaScript: /pattern/gi, Python: re.compile(pattern, re.I | re.M).

Question 1

What are regex character classes?

Accepted Answer

Regex character classes: \d matches digits 0-9, \D matches non-digits, \w matches word characters (a-z, A-Z, 0-9, _), \W matches non-word, \s matches whitespace (space, tab, newline), \S matches non-whitespace, . matches any character except newline. Use character classes to match specific character types efficiently.

Question 2

What are regex quantifiers?

Accepted Answer

Regex quantifiers control how many times a pattern matches: * means 0 or more times (a* matches empty, a, aaa), + means 1 or more (a+ matches a, aaa), ? means 0 or 1 (a? matches empty or a), {n} means exactly n times, {n,} means n or more, {n,m} means between n and m times. Quantifiers apply to the preceding character or group.

Question 3

What are regex anchors?

Accepted Answer

Regex anchors match positions, not characters: ^ matches start of string or line (with m flag), $ matches end of string or line, \b matches word boundary (between word and non-word), \B matches non-word boundary. Anchors are zero-width matches. Use ^...$ to match entire string, \b...\b to match whole words.

Question 4

What are regex groups and lookarounds?

Accepted Answer

Regex groups: (...) captures for backreference, (?:...) groups without capturing, (?=...) positive lookahead (match if followed by), (?!...) negative lookahead (match if NOT followed by), (?<=...) positive lookbehind (match if preceded), (?<!...) negative lookbehind (match if NOT preceded). Lookarounds are zero-width assertions.

Question 5

What are regex modifiers/flags?

Accepted Answer

Regex modifiers change matching behavior: i makes case-insensitive (abc matches ABC), g finds all matches not just first, m makes ^ and $ match line boundaries not just string boundaries, s makes . match newline too, x allows whitespace and comments in pattern. JavaScript: /pattern/gi, Python: re.compile(pattern, re.I | re.M).

Regex Cheat Sheet

Regex Syntax (28)

Common Patterns