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Figure A-14

When the regular expression engine reaches a position just before the

c

of

color

, it attempts to match a lower-

case

c

. This match succeeds. It next attempts to match a lowercase

o

. That too matches. It next attempts to

match a lowercase

l

and a lowercase

o

. They match as well. It then attempts to match the pattern

u?

, which

means zero or one lowercase

u

characters. Because there are exactly zero lowercase

u

characters following the

lowercase

o

, there is a match. The pattern

u?

matches zero characters. Finally, it attempts to match the final

character in the pattern — that is, the lowercase

r

. Because the next character in the string

color

does match

a lowercase

r

, the whole pattern is matched.

There is no match in the line

His collar is too tight or too colouuuurful

. The only possible match

might be in the sequence of characters

colouuuurful

. The failure to match occurs when the regular expres-

sion engine attempts to match the pattern

u?

. Because the pattern

u?

means “match zero or one lowercase u

characters,” there is a match on the first u of

colouuuurful

. After that successful match, the regular expres-

sion engine attempts to match the final character of the pattern

colou?r

against the second lowercase

u

in

colouuuurful

. That attempt to match fails, so the attempt to match the whole pattern

colou?r

against the

sequence of characters

colouuuurful

also fails.

What happens when the regular expression engine attempts to find a match in the line

These are bright

colours.

?

When the regular expression engine reaches a position just before the

c

of

colours

, it attempts to match

a lowercase

c

. That match succeeds. It next attempts to match a lowercase

o

, a lowercase

l

, and another

lowercase

o

. These also match. It next attempts to match the pattern

u?

, which means zero or one lower-

case

u

characters. Because exactly one lowercase

u

character follows the lowercase

o

in

colours

, there is

a match. Finally, the regular expression engine attempts to match the final character in the pattern, the

lowercase

r

. Because the next character in the string

colours

does match a lowercase

r

, the whole pattern

is matched.

The

findstr

utility can also be used to test for the occurrence of the sequence of characters

color

and

colour

, but the regular expression engine in the

findstr

utility has a limitation in that it lacks a metachar-

acter to signify an optional character. For many purposes, the

*

metacharacter, which matches zero, one, or

more occurrences of the preceding character, will work successfully.

327

Appendix A: Simple Regular Expressions

bapp01.qxd:bapp01 10:47 327

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