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Now that you have looked at how to match sequences of characters, each of which occur exactly once,

the next section moves on to look at matching characters that can occur a variable number of times.

Matching Optional Characters

Matching literal characters is straightforward, particularly when you are aiming to match exactly one lit-

eral character for each corresponding literal character that you include in a regular expression pattern. The

next step up from that basic situation is where a single literal character may occur zero times or one time.

In other words, a character is optional. Most regular expression dialects use the question mark (

?

) charac-

ter to indicate that the preceding chunk is optional. I am using the term “chunk” loosely here to mean the

thing that precedes the question mark. That chunk can be a single character or various, more complex reg-

ular expression constructs. For the moment, you will deal with the case of the single, optional character.

For example, suppose you are dealing with a group of documents that contain both U.S. English and

British English.

You may find that words such as

color

(in U.S. English) appear as

colour

(British English) in some

documents. You can express a pattern to match both words like this:

colou?r

You may want to standardize the documents so that all the spellings are U.S. English spellings.

Matching an Optional Character

Try this out using the Komodo Regular Expression Toolkit:

1.

Open the Komodo Regular Expression Toolkit and clear any regular expression pattern or text

that may have been retained.

2.

Insert the text

colour

into the area for the text to be matched.

3.

Enter the regular expression pattern

colou?r

into the area for the regular expression pattern.

The text

colour

is matched, as shown in Figure A-14.

Try this regular expression pattern with text such as that shown in the sample file

Colors.txt

:

Red is a color.

His collar is too tight or too colouuuurful.

These are bright colours.

These are bright colors.

Calorific is a scientific term.

“Your life is very colorful,” she said.

The word

color

in the line

Red is a color.

will match the pattern

colou?r

.

326

Appendix A: Simple Regular Expressions

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