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Forward and backward regex expressions and their JS examples

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Author: User
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definition

x(?=y)Match ' x ' only when ' x ' follows ' Y '. This is called positive lookup .

For example,/jack (? =sprat)/Will match to ' Jack ' only when it is followed by ' Sprat '. /jack (? =sprat| Frost)/Match ' Jack ' only when it is followed by ' sprat ' or ' Frost '. But ' sprat ' and ' Frost ' are not part of the matching results.

x(?!y)Match ' x ' only when ' X ' is not followed by ' Y ', this is called positive negation lookup.

For example,/\d+ (?! \.) /Match a number only when the number is not followed by a decimal point. Regular expression/\d+ (?! \.) /.exec ("3.141") matches ' 141 ' but not ' 3.141 '

Form Https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Guide/Regular_Expressions

See http://www.cnblogs.com/dolphinx/p/3486214.html explain the comparison understood.

Example 
<HTML><Head> </Head> <Body>    <inputID= "Test"type= "text"value="" />    <inputID= "Test"type= "text"value="" />    <inputID= "Test"type= "text"value="" />    <inputID= "Test"type= "text"value="" />    <inputID= "Test"type= "text"value="" />    <Script>    varTeststr= "Windows"        /*1-no band expression match*/    varTestreg= /^windows. *$/    varresult=Teststr.match (Testreg); Console.log ("/^windows. *$/="+result)///^windows. *$/=windows        /*2-band expression matching*/    varTestreg= /^windows (. *) $/    varresult=Teststr.match (Testreg); Console.log ("/^windows (. *) $/="+result)///^windows (. *) $/=windows 95,95        /*3-A band expression that does not record its matching results*/    varTestreg= /^windows (?:. *) $/    varresult=Teststr.match (Testreg); Console.log ("/^windows (?:. *) $/="+result)///^windows (?:. *) $/=windows        /*4-forward match, match position, positive match*/    varTestreg= /^windows (? =95) 95$/    varresult=Teststr.match (Testreg); Console.log ("/^windows (? =.*) $/="+result)///^windows (=.*) $/=windows    /*5-forward match, match position, negative match*/    varTeststr= "Windows Me"    varTestreg= /^windows (?! me$)/    varresult=Teststr.match (Testreg); Console.log ("/^windows (?! \d*) $/="+result)///^windows (?! d*) $/=windows me        </Script></Body></HTML>

Forward and backward regex expressions and their JS examples

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