Repetition operators in regular expressions

On the previous page, we mentioned the dot and asterisk combination, meaning "any character repeated any times". The asterisk is what is sometimes called a repetition operator.

How repetition operators work

Repetition operators are placed after the item to be repeated. We have already seen that .* means any character repeated any number of times. By item, we mean a single character or a character class. So for example, [0-9]* means any number of digits and a* means any number of instances of the letter a.

Available repetition operators

The following operators all behave in a similar way to the asterisk: they are placed after the repeated item.

Operator	Meaning	Examples
`*`	Zero or more...	`.` any character sequence* `[0-9]` any number of digits* `a` any number of letter `a`s*
`?`	Zero or one... (i.e. optional element)	`[0-9]?` An optional digit `X?` An optional letter X
`+`	One or more...	`[0-9]+` One or more digits
`{x}`	x instance of...	`[0-9]{10}` Ten digits `n{4}` Four instances of the letter n `.{3}` Three instances of any character
`{x,y}`	between x and y instances of...	`[0-9]{10,14}` Between 10 and 14 digits
`{x,}`	at least x instances of...	`.{5,}` At least 5 characters

Now you can understand why the following expression:

.*[0-9]{10}.*

matches a string containing ten digits.

Resolving ambiguity: greedy and reluctant operators

In an expression such as that above, there are potential ambiguities. For example, given a string with more than ten characters, how do we decide which .* matches against the extra digits? And what stops the first .* from simply matching against the entire string, digits or not? On the next page we answer these questions by looking at the notion of greedy and reluctant operators.