PHP中的正规表达式(二)
确定重复出现
到现在为止,你已经知道如何去匹配一个字母或数字,但更多的情况下,可能要匹配一个单词或一组数字。一个单词有若干个字母组成,一组数字有若干个单数组成。跟在字符或字符簇后面的花括号({})用来确定前面的内容的重复出现的次数。
字符簇 含义
^[a-zA-Z_]$ 所有的字母和下划线
^[[:alpha:]]{3}$ 所有的3个字母的单词
^a$ 字母a
^a{4}$ aaaa
^a{2,4}$ aa,aaa或aaaa
^a{1,3}$ a,aa或aaa
^a{2,}$ 包含多于两个a的字符串
^a{2,} 如:aardvark和aaab,但apple不行
a{2,} 如:baad和aaa,但Nantucket不行
\t{2} 两个制表符
.{2} 所有的两个字符
这些例子描述了花括号的三种不同的用法。一个数字,{x}的意思是“前面的字符或字符簇只出现x次”;一个数字加逗号,{x,}的意思是“前面的内容出现x或更多的次数”;两个用逗号分隔的数字,{x,y}表示“前面的内容至少出现x次,但不超过y次”。我们可以把模式扩展到更多的单词或数字:
^[a-zA-Z0-9_]{1,}$ //所有包含一个以上的字母、数字或下划线的字符串
^[0-9]{1,}$ //所有的正数
^\-{0,1}[0-9]{1,}$ //所有的整数
^\-{0,1}[0-9]{0,}\.{0,1}[0-9]{0,}$ //所有的小数
最后一个例子不太好理解,是吗?这么看吧:与所有以一个可选的负号(\-{0,1})开头(^)、跟着0个或更多的数字([0-9]{0,})、和一个可选的小数点(\.{0,1})再跟上0个或多个数字([0-9]{0,}),并且没有其他任何东西($)。下面你将知道能够使用的更为简单的方法。
特殊字符"?"与{0,1}是相等的,它们都代表着:“0个或1个前面的内容”或“前面的内容是可选的”。所以刚才的例子可以简化为:
^\-?[0-9]{0,}\.?[0-9]{0,}$
特殊字符"*"与{0,}是相等的,它们都代表着“0个或多个前面的内容”。最后,字符"+"与 {1,}是相等的,表示“1个或多个前面的内容”,所以上面的4个例子可以写成:
^[a-zA-Z0-9_]+$ //所有包含一个以上的字母、数字或下划线的字符串
^[0-9]+$ //所有的正数
^\-?[0-9]+$ //所有的整数
^\-?[0-9]*\.?[0-9]*$ //所有的小数
当然这并不能从技术上降低正规表达式的复杂性,但可以使它们更容易阅读。

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