regex

Overview

Regular expressions describe text patterns for matching, extraction, validation, and replacement. Start with the Regex basics guide for engine differences, groups, quantifiers, boundaries, and testing, then use the focused references below.

16 pages

• regex alternation — | operator and grouping

  Use | in regex to match one of several alternatives. Covers grouping with (), non-capturing groups (?:...), and alternation precedence.

• regex anchors — ^ and $ match start and end

  Use ^ to match the start and $ to match the end of a string or line in regex. Covers \A, \Z in Python and the multiline flag.

• regex character classes — [...], \d, \w, \s

  Use regex character classes to match sets of characters. Covers [...], negated [^...], shorthand classes \d \w \s and their inverses.

• regex match date — YYYY-MM-DD and common formats

  Regex patterns to match ISO 8601 dates (YYYY-MM-DD) and other common date formats. Covers validation and extraction in JavaScript and Python.

• regex email pattern — validate and extract email addresses

  Regex pattern to validate or extract email addresses. Covers basic validation, RFC 5322, and practical patterns for JavaScript and Python.

• regex global replace — replace all matches in JavaScript, Python, sed

  Replace all regex matches in a string using the global flag in JavaScript, re.sub() in Python, or sed s///g. Covers case-insensitive and multiline replacements.

• regex match IPv4 address

  Regex pattern to match and validate IPv4 addresses (0-255 per octet). Covers simple and strict patterns in JavaScript and Python.

• regex lookahead and lookbehind — (?=...) (?<=...) (?!...) (?<!...)

  Use regex lookahead (?=...) and lookbehind (?<=...) assertions to match patterns without including them in the result.

• regex multiline flag — ^ and $ match line start/end

  Use the multiline flag (m) so ^ and $ match the start and end of each line, not just the entire string. Covers JavaScript, Python, and the DOTALL flag.

• regex named capture groups — (?P<name>) and (?<name>)

  Use named capture groups in regex to extract named parts of a match. Covers Python (?P<name>), JavaScript (?<name>), and backreferences.

• regex non-greedy matching — .*? vs .*

  Use non-greedy (lazy) quantifiers like .*? to match as little as possible. Explains the difference between greedy and lazy matching with examples.

• regex match phone number — international and US formats

  Regex patterns to match phone numbers in US, international (+1-XXX), and flexible formats. Covers extraction and normalization.

• Regex basics — patterns, groups, quantifiers, boundaries, and safe matching

  Understand regular expression literals, character classes, groups, quantifiers, anchors, boundaries, flags, replacement, and safe pattern testing across regex engines.

• regex replace with capture group backreference

  Use capture group backreferences ($1, \1, \g<1>) in regex replacements to reuse matched text. Covers JavaScript, Python, sed, and named groups.

• regex match URL — http, https, and optional path

  Regex patterns to match and extract URLs (http/https) from text in JavaScript and Python. Covers optional paths, query strings, and fragments.

• regex word boundary \b — match whole words only

  Use \b word boundary assertion to match whole words and avoid partial matches. Covers \b, \B, and word boundary in different languages.