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stage, the first three characters in the regular expression pattern have been matched. Finally, an attempt
is made to match the pattern
[0-9]*
, which means “Match zero or more numeric characters.” Because
the character after
C
is a newline character, there are no numeric digits. Because there are exactly zero
numeric digits after the uppercase
C
of
ABC
, there is a match (of zero numeric digits). Because all compo-
nents of the pattern match, the whole pattern matches.
Why does the part number
ABC8899
also match? When the regular expression engine is at the position
immediately before the
A
of
ABC8899
, it attempts to match the next character in the part number with an
uppercase
A
. Because the first character of the part number
ABC8899
is an uppercase
A
, there is a match.
Next, attempts are made to match an uppercase
B
and an uppercase
C
. These too match. At that stage, the
first three characters in the regular expression pattern have been matched. Finally, an attempt is made to
match the pattern
[0-9]*
, which means “Match zero or more numeric characters.” Four numeric digits
C
. Because there are exactly four numeric digits after the uppercase
C
of
ABC
, there
is a match (of four numeric digits, which meets the criterion “zero or more numeric digits”). Because all
components of the pattern match, the whole pattern matches.
Work through the other part numbers step by step, and you’ll find that each ought to match the pattern
ABC[0-9]*
.
The + Quantifier
There are many situations where you will want to be certain that a character or group of characters
is present at least once but also allow for the possibility that the character occurs more than once. The
+
cardinality operator is designed for that situation. The
+
operator means “Match one or more occur-
rences of the chunk that precedes me.”
Take a look at the example with
Parts.txt
, but look for matches that include at least one numeric digit.
You want to find part numbers that begin with the uppercase characters
ABC
and then have one or more
numeric digits.
You can express the problem definition like this:
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 one or
more numeric digits.
Use the following pattern to express that problem definition:
ABC[0-9]+
Matching One or More Numeric Digits
1.
Open OpenOffice.org Writer, and open the sample file
Parts.txt
.
2.
Use Ctrl+F to open the Find And Replace dialog box.
3.
Check the Regular Expressions and Match Case check boxes.
4.
Enter the pattern
ABC[0-9]+
in the Search For text box; click the Find All button, and inspect
the matching part numbers that are highlighted, as shown in Figure A-18.
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Appendix A: Simple Regular Expressions
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