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Other Cardinality Operators

Testing for matches only for optional characters can be very useful, as you saw in the

colors

example,

but it would be pretty limiting if that were the only quantifier available to a developer. Most regular

expression implementations provide two other cardinality operators (also called

quantifiers

): the

*

operator and the

+

operator, which are described in the following sections.

The * Quantifier

The

*

operator refers to zero or more occurrences of the pattern to which it is related. In other words,

a character or group of characters is optional but may occur more than once. Zero occurrences of the

chunk that precedes the

*

quantifier should match. A single occurrence of that chunk should also match.

So should two occurrences, three occurrences, and ten occurrences. In principle, an unlimited number of

occurrences will also match.

Try this out in an example using OpenOffice.org Writer.

Matching Zero or More Occurrences

The sample file,

Parts.txt

, contains a listing of part numbers that have two alphabetic characters followed

by zero or more numeric digits. In the simple sample file, the maximum number of numeric digits is three,

but because the

*

quantifier will match three occurrences, you can use it to match the sample part numbers. If

there is a good reason why it is important that a maximum of three numeric digits can occur, you can express

that notion by using an alternative syntax, which is discussed a little later in this appendix. Each of the part

numbers in this example consists of the sequence of uppercase characters

ABC

followed by zero or more

numeric digits:

ABC

ABC123

ABC12

ABC889

ABC8899

ABC34

You can express what you want to do as follows:

Match an uppercase

A

. If there is a match, attempt to match an uppercase

B

. If there is a match,

attempt to match an uppercase

C

. If all three uppercase characters match, attempt to match zero or

more numeric digits.

Because all the part numbers begin with the literal characters

ABC

, you can use the pattern

ABC[0-9]*

to match part numbers that correspond to the description in the problem definition.

1.

Open OpenOffice.org Writer and open the sample file,

Parts.txt

.

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Appendix A: Simple Regular Expressions

bapp01.qxd:bapp01 10:47 332

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