Regex Character Classes, Quantifiers & Anch...

Most regular expressions are built from three kinds of piece: what to match (character classes), how many (quantifiers), and where (anchors). Master these and the rest of regex is mostly combination.

This is a supporting guide in The Complete Guide to Regular Expressions.

Character classes

A character class matches exactly one character from a set:

[abc] — an a, b, or c
[a-z], [A-Z], [0-9] — ranges
[a-zA-Z0-9_] — combine ranges and characters
[^...] — negate: [^0-9] is any non-digit

Inside a class most metacharacters lose their special meaning — [.] matches a literal dot, no escaping needed. The characters that still matter inside a class are ^ (only first), - (a range unless first/last), and ].

Shorthand classes

Common sets have shortcuts you'll use constantly:

Shorthand	Matches	Equivalent
`\d`	a digit	`[0-9]`
`\w`	a word char	`[A-Za-z0-9_]`
`\s`	whitespace	space, tab, newline
`\D` `\W` `\S`	the negations	not the above

Quantifiers

Quantifiers repeat the item just before them:

* — zero or more
+ — one or more
? — zero or one (optional)
{3} — exactly three
{2,5} — between two and five
{2,} — two or more

Greedy vs lazy

By default quantifiers are greedy: they match as much as they can, then give back if needed. Add ? to make them lazy (match as little as possible). The classic example is matching inside tags:

Input:  <b>one</b><b>two</b>
<.+>    -> matches the whole line (greedy)
<.+?>   -> matches <b> then </b> ... (lazy, each tag)

Reaching for lazy quantifiers is often the fix when a pattern "matches too much".

Anchors and boundaries

Anchors match a position between characters, not a character itself:

^ — start of the string (or line, with the m flag)
$ — end of the string (or line)
\b — a word boundary (between \w and non-\w)
\B — not a word boundary

^\d{3}-\d{4}$ means "the entire string is 3 digits, a dash, then 4 digits" — the anchors are what stop it from matching that pattern inside a longer string.

Part of The Complete Guide to Regular Expressions. See also Groups, Backreferences & Lookarounds.

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Regex Character Classes, Quantifiers & Anchors