October 2022
Beginner to intermediate
390 pages
7h 5m
Chinese
Content preview from Perl语言入门（第8版）
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140
｜
第
7
章
现在我们需要转义的元字符又多了这些：
?
、
*
、
+
和
{
。在
Perl 5.26
之前，左花括号
（
{
）有时不用转义也可以表示该字符本身，但随着
Perl
的沿革，正则表达式的特性
也在扩展，
{
被赋予了更多能力和使命，所以在之后的版本中都需要转义，才能表示
它本身。相比之下，右花括号因为不会开启任何特殊处理，也就不需要转义了。
Perl 5.28
暂时放宽了将特殊字符
{
进行转义的要求，给用户更多时间来修复遗
留代码。
Perl 5.30
则重新开启了这个要求。
所有以上这些量词都可以用另一种统一方式重新书写，我们列在表
7-1
中。
表
7
-
1
：正则表达式量词及其对应的广义形式
匹配次数
元字符
一般化写法
可有可无
? {0,1}
零或多次
* {0,}
一次以上
+ {1,}
至少多少次，不设上限
{3,}
指定重复次数范围
{3,5}
指定可以不匹配的重复次数范围
{0,5}
或
{,5}(5.34
版
)
准确的重复次数
{3}
模式分组
我们可以用圆括号
()
将模式字符串分组，所以圆括号也是元字符。
我们知道，量词只作用于它之前的那个字符，像模式
/fred+/
能匹配
freddddddddd
这
样的字符串，因为量词
+
之前的是字母
d
。如果要匹配整个
fred
的重复次数，就得
用括号把它围起来，写作
/(fre d)+/
，此时量词对应要重复的就是前面这个用括号围
起来的部分，所以能匹配
fredfredfred
这样的字符串。
圆括号还能帮我们在表达式内重复利用匹配到的字符。我们可以用反向引用（ ...
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I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.