The ambiguity of parentheses in regular expressions. Regular expressions. Remove parentheses. Regular expressions. Matching parentheses. Regular expressions. Matching parentheses.

Parentheses in regular expressions have the following meanings

• Limit the scope of the quantifier
• Limit the scope of the multi-select structure
• Capture text for back references
• Group capture
• Only grouping but not capturing
• Look ahead

1. Limit the scope of quantifiers

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varreg1 = /(Matz)?/; // 0 or 1 Matz var reg2 = /(Matz)+/; // 1 or more Matz var reg3 = /(Matz)*/; // 0 or more Matz

2. Limit the range of the multi-select structure

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varreg = /(Matz|Eich)/ reg.test('Matz') // => true reg.test('Eich') // => true reg.test('John') // => false

3. Capture text for backreferences

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varreg = /(boy)1/ // Equivalent to /boyboy/ reg.test('boy') // => false reg.test('boyboy') // => true var reg /(boy)(girl)12/ reg.test('boygirlboygirl') // => true

Four. Group capture

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var reg1 = /(d{3}) (d{3})/ var str = '111 222' str.replace(reg1, '$2 $1')// => '222 111' , note that $2 and $1 here store the matching string var reg2 = /(d{3})(d{4})(d{4})/ var mobile = '13522722724' reg2.test(mobile) RegExp.$1 // => 135 RegExp.$2 // => 2272 RegExp.$3 // => 2724 var reg3 = /(d{3})(d{4})(d{4})/ var mobile = '13522722724' mobile.replace(reg3, '$1 $2 $3')// => '135 2272 2724'

5. Only grouping without capturing (together with "?:")

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var reg = /(?:d+)/ reg.test('13522722724') RegExp.$1 // => '' Do not store matched elements

In longer regular expressions, back references will reduce the matching speed and performance. Grouping without capturing should be used when back references are not needed. .

6. Lookahead (with "?=")

It tells the regular expression to look forward some characters but not move the position. Lookahead does not match any characters and only matches specific positions in the text.

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var reg = /(John) (?=Resig)/ reg.test('John') // => false reg.test('John Backus')// => false reg.test('John Reisg')// => true RegExp.$1 // => 'John', please note that this is not "John Resig"

The following is a small function that uses lookahead to format mobile phone numbers

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/* * Separate mobile phone numbers * 13522722724 -> 135 2272 2724 */ function separateMobile(num) { var arr = ('0'+ num ).replace(/(d{4})(?=d)/g,"$1 ").split('') arr.shift() return arr.join('' ) }

The above introduces the ambiguity of regular expression parentheses, including regular expressions and parentheses. I hope it will be helpful to friends who are interested in PHP tutorials.