Reverse pre-search (top) in Regular Expression)

In the past, most of the applications were pre-searches. That is to say, there are specific content limits on the right side of the search content, for example, the following example: CopyCode The Code is as follows: var str3 = 'cpu: Intel core2 5200; Memory: 2G; OS: Ubuntu 9.04 ';
VaR reg3 =/memory \: \ s + (.*(? = \; OS \ :))/Gim;
Str3.match (reg3 );
Alert (Regexp. $1); // The result is 2G

(.*(? = \; OS \ :) is a typical forward pre-search. Only "; OS:" On the right will be matched.
however, the content on the left is fixed, not on the right. However, JavaScript does not support reverse pre-search. I believe that careful friends have discovered that the above example has achieved this goal (the previous memory \: \ s + limits the content on the left ). The following is an example: copy Code the code is as follows: // Program , remove the domain name
var STR = '';
var reg1 =/(\ Str. match (reg1);
alert (Str. replace (Regexp. $4, '');

Is there any reverse pre-search? The answer is yes, but the reality is cruel. I checked some materials. The reverse pre-search format mentioned above is as follows? <= Or? <! For guidance. Unfortunately, JavaScript is not supported and has been supported in the high version of Java, so I wrote the following test program (jre1.6.0 _ 03 ):Copy codeThe Code is as follows: Import java. util. RegEx. matcher;
Import java. util. RegEx. pattern;
Public class regexptest {
Public static void main (string [] ARGs ){
Pattern P = pattern. Compile ("((? <=\\ Matcher = P. matcher (" ");
System. Out. println (matcher. Matches ());
}
}

The program itself should have no problems, but the running results:

Exception in thread "Main" Java. util. RegEx. patternsyntaxexception: Look-behind group does not have an obvious maximum length near index 27
((? <=\

I spent half a day on Google, and the only result I got was a new one. Hope to see thisArticleIf you have already achieved this effect, you must leave a message for us.

What else do I see when I read the document? : The guiding regular expression, which says "matching is not stored", cannot be understood. I feel like the code is true. Let's take a look:Copy codeThe Code is as follows: var str2 = 'client name ';
VaR reg2 =/(client name (? )?) /;
Str2.match (reg2 );
Alert (Regexp. $1); // client name
Alert (Regexp. $2); // empty string
// Do not use? :
Reg2 =/(client name (name )?) /;
Str2.match (reg2 );
Alert (Regexp. $1); // client name
Alert (Regexp. $2); // name

In Agile development, code is the best document. Gave me an excuse.