Pattern Basics

Regex Pattern Basics

A regular expression is a compact pattern that describes text. Most engines share the same core syntax: literals, wildcards, character classes, quantifiers, and anchors.

Literals and the Dot

Most characters match themselves literally. The dot . is the wildcard — it matches any single character except a newline by default.

/c.t/   matches "cat", "cot", "c9t" — not "ct"

Character Classes

Square brackets define a set of allowed characters. A range uses a hyphen, and a leading ^ inside the brackets negates the set.

/[aeiou]/    any vowel
/[a-z]/      any lowercase letter
/[^0-9]/     anything that is NOT a digit

Common classes have shorthand forms: \d (digit), \w (word character), \s (whitespace). Their uppercase versions negate them.

Quantifiers

Quantifiers control how many times the preceding element repeats.

• * — zero or more
• + — one or more
• ? — zero or one (optional)
• {n,m} — between n and m times

/colou?r/      "color" or "colour"
/\d{3}-\d{4}/  "555-1234"
/a+/           "a", "aa", "aaa", ...

Greedy vs Lazy

By default quantifiers are greedy: they match as much as possible. Adding ? after a quantifier makes it lazy, matching as little as possible.

/<.*>/    greedy on "<a><b>" matches the WHOLE string
/<.*?>/  lazy matches just "<a>"

Greedy matching is a frequent source of surprises. When parsing tags or delimited fields, reach for the lazy *? or +? — it usually gives the match you actually wanted.

Anchors

Anchors do not consume characters — they assert a position. ^ matches the start of the string; $ matches the end.

/^\d+$/   the entire string must be digits
/^Hello/  the string must start with "Hello"