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.
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.
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 as |
| ? | 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.
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.
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Editorial page content written by Neil Coffey. Copyright © Javamex UK 2021. All rights reserved.