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Character classes: one of these

Square brackets describe a single character by listing what it may be.

Lesson 2 of 4 · about 3 minutes

Brackets match exactly one character

Square brackets describe one character by listing the options. [aeiou] matches a single vowel — not a run of them. This trips people up often enough to be worth stating twice: a class is one character, however many you list inside it.

Ranges are shorthand for a run of the character set: [a-z] is every lowercase letter, [0-9] every digit, [a-zA-Z0-9] any letter or digit. The dash only means a range between two characters; first or last in the brackets it is just a dash, which is why [-a-z] is a valid way to include one.

Negation, and the trap in it

A caret immediately after the opening bracket inverts the class: [^0-9] matches any single character that is not a digit. Anywhere else in the brackets a caret is an ordinary caret.

The trap is that a negated class still has to match something. [^0-9] does not mean "no digit here" — it means "one character, and it is not a digit". Against the text "42" it fails at both positions, but against "4a" it matches the a.

The shorthands, and what they quietly include

\d is a digit, \w is a word character, \s is whitespace, and each has an inverse: \D, \W, \S. They are shorter and they read better, and they are worth knowing precisely.

\w is not "a letter". It is [A-Za-z0-9_] — digits and the underscore included, accented letters excluded. So \w+ will happily match "user_2" and will stop short on "café". If you are validating names, that matters; JavaScript's u flag with Unicode property escapes such as \p{L} is the honest tool for letters in any script.

\s covers more than a space: tabs, newlines, carriage returns and several Unicode spaces. That is usually what you want when trimming, and occasionally a surprise when you meant a literal space.

Worked examples

  • Match a single hex digit

    /[0-9a-fA-F]/g

    Against: Colour #3fB2z9

    Every character matches individually here except z — a class is one character at a time.

  • See what \w leaves out

    /\w+/g

    Against: user_2 café naïve

    It matches user_2 in full, then caf, then na — the accented letters are outside \w.

Check yourself

Practice questions written for this lesson — not past exam papers.

Practise every question in this subject

  1. How many characters does [abc] match?

  2. Which of these does \w match?

Try it