Repetition - Page 9

We've now moved from matching a specific character to a more general type of character - when we don't know (or don't care) exactly what the character will be. Now we're going to see what happens when we want to talk about a more general quantity of characters: more than three digits in a row; two to four capital letters, and so on. The metacharacters that we use to deal with a number of characters in a row are called quantifiers.

Indefinite Repetition

The easiest of these is the question mark. It should suggest uncertainty - something may be there, or it may not. That's exactly what it does: stating that the immediately preceding character(s) - or metacharacter(s) - may appear once, or not at all. It's a good way of saying that a particular character or group is optional. To match the word 'he or she', you can put:

> perl matchtest.plx
Enter some text to find: \bs?he\b
The text matches the pattern '\bs?he\b'.
 >

To make a series of characters (or metacharacters) optional, group them in parentheses as before. Did he say 'what the Entish is' or 'what the Entish word is'? Either will do:

> perl matchtest.plx
Enter some text to find: what the Entish (word )?is
The text matches the pattern 'what the Entish (word )?is'.
>

Notice that we had to put the space inside the group: otherwise we end up with two spaces between 'Entish' and 'is', whereas our text only has one:

> perl matchtest.plx
Enter some text to find: what the Entish (word)? is
'what the Entish (word)? is' was not found.
 >

As well as matching something one or zero times, you can match something one or more times. We do this with the plus sign - to match an entire word without specifying how long it should be, you can say:

> perl matchtest.plx
Enter some text to find: \b\w+\b
The text matches the pattern '\b\w+\b'.
 >

In this case, we match the first available word - I.

If, on the other hand, you have something which may be there any number of times but might not be there at all - zero or one or many - you need what's called 'Kleene's star': the * quantifier. So, to find a capital letter after any - but possibly no - spaces at the start of the string, what would you do? The start of the string, then any number of whitespace characters, then a capital:

> perl matchtest.plx
Enter some text to find: ^\s*[A-Z]
'^\s*[A-Z]' was not found.
  
>

Of course, our test string begins with a quote, so the above pattern won't match, but, sure enough, if you take away that first quote, the pattern will match fine. Let's review the three qualifiers:

/bea?t/	Matches either 'beat' or 'bet'
/bea+t/	Matches 'beat', 'beaat', 'beaaat'…
/bea*t/	Matches 'bet', 'beat', 'beaat'…

Novice Perl programmers tend to go to town on combinations of dot and star, and the results often surprise them, particularly when it comes to searching-and-replacing. We'll explain the rules of the regular expression matcher shortly, but bear the following in mind:

A regular expression should hardly ever start or finish with a starred character.

You should also consider the fact that .* and .+ in the middle of a regular expression will match as much of your string as they possibly can. We'll look more at this 'greedy' behavior later on.

Posix and Unicode Classes - Page 8
Beginning Perl
Well-Defined Repetition - Page 10