Basic Regex Syntax

Regular expressions (regex) are patterns used to match character combinations in strings. Here are the basics:

• . - Matches any character except a newline
• \d - Matches any digit (0-9)
• \w - Matches any word character (a-z, A-Z, 0-9, _)
• \s - Matches any whitespace character
• [abc] - Matches any character in the set (a, b, or c)
• [^abc] - Matches any character not in the set (not a, b, or c)
• {n} - Matches exactly n occurrences
• * - Matches 0 or more occurrences
• + - Matches 1 or more occurrences
• ^ - Anchors to the start of a string
• $ - Anchors to the end of a string

Regex Quantifiers

Quantifiers specify how many times a pattern should occur:

• * - Matches 0 or more times
• + - Matches 1 or more times
• ? - Matches 0 or 1 time (optional)
• {n} - Matches exactly n times
• {n,} - Matches n or more times
• {n,m} - Matches between n and m times

Regex Anchors

Anchors ensure that your pattern matches specific positions in the string:

• ^ - Matches the beginning of a string
• $ - Matches the end of a string
• \b - Word boundary
• \B - Not a word boundary

Regex Groups and Lookahead

Groups and lookaheads are useful for capturing parts of matches or checking for conditions without consuming characters:

• (abc) - Capturing group for abc
• (?:abc) - Non-capturing group
• (?=abc) - Positive lookahead (asserts that abc is ahead)
• (?!abc) - Negative lookahead (asserts that abc is not ahead)

Character Classes

Character classes allow you to match specific sets of characters:

• [abc] - Matches "a", "b", or "c"
• [^abc] - Matches any character except "a", "b", or "c"
• [a-z] - Matches any lowercase letter
• [A-Z] - Matches any uppercase letter
• [0-9] - Matches any digit

Common Regex Patterns