August 2018
Beginner to intermediate
382 pages
7h 14m
Chinese
Content preview from Perl语言入门
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134
｜
第
7
章
后面谈到字符集简写时我们会看到更多
\N
的用法。
量词
我们可以用量词指定匹配项的重复次数。量词也是元字符，它把写在它之前的字符作
为要匹配的目标，然后定义它该重复匹配的次数。也有人称之为重复次数，或重复操
作符。
最简单的量词是问号
?
，它的意思是，前一个字符可以出现一次或干脆不出现（或
者说，前一个字符可有可无）。比如有的人写成
Bamm-bamm
，而有的人写成没有连字
符的
Bammbamm
，而你希望对两种写法都能匹配，那么这里的连字符
-
就是可有可无
的：
$_ = 'Bamm-bamm';
if (/Bamm-?bamm/) {
print "It matched!\n";
}
用你的测试程序试试看，以不同的方式输入
Bamm-Bamm
的名字会得到什么结果：
%
perl try_a_pattern "Bamm-?bamm"
Bamm-bamm
Matches
Bammbamm
Matches
Are you Bammbamm or Bamm-bamm?
Matches
仔细观察输入的最后一行，到底哪个版本得到了可以匹配的结论？
Perl
从目标字符串
最左端开始扫描，逐个判断当前位置上是否能达成匹配，直到发现符合条件的字符为
止，然后结束工作。所以这行匹配的是前一个版本，即
Bammbamm
。一旦
Perl
完成模
式匹配，就不再继续下去了，哪怕后面还有可以匹配的项。
Perl
总是匹配最左端能找
到的第一个子字符串，不管后续还有无匹配的，都不会改变当前结论，没必要做无用
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