


In-depth understanding of the analysis of grouping of JS regular expressions
这篇文章主要介绍了关于深入理解JS正则表达式之分组的解析,有着一定的参考价值,现在分享给大家,有需要的朋友可以参考一下
分组
在使用正则的时候,有时候会想要匹配一串字符串连续出现多次的情况,比如:我想匹配字符串Byron
连续出现3次的情况。
有些人会直接写:
Byron{3}
但是,这种情况仅仅会匹配Byro
加上三个n
,显然,这样是错误的。
要想实现之前的需求,我们需要使用正则表达式的分组功能:使用()
可以达到分组的功能,使量词作用于分组。所以,如下正则可以顺利匹配Byron
三次
(Byron){3}
或
JS中的|
的含义是或,在正则中,我们也可以使用|
达成或的效果。比较常用的或的写法是:
Byron | Casper
即,Byron或Casper二选一。
还有一种用法是配合分组使用:
let text = 'ByronsperByrCasper' let reg = /Byr(on|Ca)sper/g // 匹配Byronsper或ByrCasper text.replace(reg, 'X') // XX
反向引用
此时,我们有一个需求,将一种格式的时间字符串:yyyy-MM-DD
转为MM/DD/yyyy
类型格式字符串。
这个问题的难点在于,不仅要将-
转为/
还要将年月日的位置进行调换。由于年月日的数值是不固定的,没法直接转换为固定数值。这时,我们可以使用反向引用解决这个问题。
反向引用是分组中的一个重要的高级语法,主要用来解决分组变量的问题。
什么叫分组变量?分组变量就是通过分组匹配到的值,正则表达式将其转为了一种可以被调用的变量。而调用这个变量也很简单$n
n代表着分组的序号,值得一提的是,序号是从1开始,而不是从0开始。
通过反向引用可以很轻松的解决上面那个问题:
let text = '2018-07-09' let reg = /(\d{4})-(\d{2})-(\d{2})/g /* $1是(\d{4})的匹配内容,代表yyyy $2是第一个(\d{2})的匹配内容,代表MM $3是第二个(\d{2})的匹配匹配内容,代表DD */ text.replace(reg, '$2/$3/$1') // 07/09/8
忽略分组
当不希望捕获某些分组,只需要在分组内加上?:
即可以
(?:Byron).(ok)
以上就是本文的全部内容,希望对大家的学习有所帮助,更多相关内容请关注PHP中文网!
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