Regular Expressions

A regular expression (regex) is a sequence of characters that defines a search pattern. They are used to match, search, and extract text in virtually every programming language.

Here are the fundamental building blocks:

• abc — Literal characters match themselves exactly
• . — Wildcard matches any single character (except newline)
• \d — Matches any digit (0-9)
• \w — Matches any word character (letter, digit, or underscore)
• \s — Matches any whitespace (space, tab, newline)

Try typing patterns in the tester below. Start with something simple like cat or \d:

Quantifiers control how many times a character or group is matched:

• * — Zero or more times (e.g. go*d matches "gd", "god", "good")
• + — One or more times (e.g. go+d matches "god", "good", but not "gd")
• ? — Zero or one time (e.g. colou?r matches "color" and "colour")
• {n} — Exactly n times (e.g. \d{3} matches exactly 3 digits)

Anchors match positions, not characters:

• ^ — Start of string
• $ — End of string

Try \d{3}-\d{4} to match phone numbers, or ^The to match "The" only at the start:

Character classes match one character from a set:

• [abc] — Matches "a", "b", or "c"
• [a-z] — Matches any lowercase letter
• [A-Z0-9] — Matches any uppercase letter or digit
• [^abc] — Matches anything except "a", "b", or "c"

Groups let you capture and group parts of a pattern:

• (group) — Captures the matched text as a group
• a|b — Alternation matches "a" or "b"

Try (cat|dog) to match either word, or [aeiou] to find all vowels:

1. Which pattern matches any single digit?

2. What does the + quantifier mean?

3. Which pattern matches either "cat" or "dog"?