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attempts to match an optional lowercase

s

— in other words, to match zero or one occurrence of lower-

case

s

. Because there is no occurrence of lowercase

s

, again, there is a match. Finally, an attempt is made

to match an optional apostrophe. Because there is no occurrence of an apostrophe, another match is

found. Because a match exists for all the components of the regular expression pattern, there is a match

for the whole regular expression pattern

colour?r’?s?’?

.

Now, how does the pattern

colou?r’?s?’?

match the word

colour

? Assume that the regular expression

engine is at the position immediately before the first letter of

colour

. It first attempts to match lowercase

c

,

because one lowercase

c

must be matched. That matches. Next, attempts are made to match a subse-

quent lowercase

o

,

l

, and another

o

. These also match. Then an attempt is made to match an optional

lowercase

u

. In other words, zero or one occurrences of the lowercase character

u

are needed. Because

there is one occurrence of lowercase

u

, there is a match. Next, an attempt is made to match lowercase

r

.

The lowercase

r

in

colour

matches. Next, the engine attempts to match an optional apostrophe. Because

there is no occurrence of an apostrophe, there is a match. Next, the regular expression engine attempts

to match an optional lowercase

s

— in other words, to match zero or one occurrences of lowercase

s

.

Because there is no occurrence of lowercase

s

, a match exists. Finally, an attempt is made to match an

optional apostrophe. Because there is no occurrence of an apostrophe, there is a match. All the compo-

nents of the regular expression pattern have a match; therefore, the entire regular expression pattern

colour?r’?s?’?

matches.

Work through the other six word forms shown earlier, and you’ll find that each of the word forms does,

in fact, match the regular expression pattern.

The pattern

colou?r’?s?’?

matches all eight of the word forms that were listed earlier, but will the

pattern match the following sequence of characters?

colour’s’

Can you see that it does match? Can you see why it matches the pattern? If each of the three optional

characters in the regular expression is present, the preceding sequence of characters matches. That rather

odd sequence of characters likely won’t exist in your sample document, so the possibility of false matches

(reduced specificity) won’t be an issue for you.

How can you avoid the problem caused by such odd sequences of characters as

colour’s’

? You want

to be able to express it something like this:

Match a lowercase

c

. If a match is present, attempt to match a lowercase

o

. If that match is

present, attempt to match a lowercase

l

. If there is a match, attempt to match a lowercase

o

.

If a match exists, attempt to match an optional lowercase

u

. If there is a match, attempt to

match a lowercase

r

. If there is a match, attempt to match an optional apostrophe. And if

a match exists here, attempt to match an optional lowercase

s

. If the earlier optional apostrophe

was not present, attempt to match an optional apostrophe.

With the techniques that you have seen so far, you aren’t able to express ideas such as “match

something only if it is not preceded by something else.” That sort of approach might help achieve

higher specificity at the expense of increased complexity.

331

Appendix A: Simple Regular Expressions

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