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:
Copy codeThe 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 ). Here is another example:
Copy codeThe Code is as follows:
// Program purpose, remove the domain name in the image path
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. If you want to see this article, you must leave a message to 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.