Regular Expressions: Backreferences

You can refer to the submatches resulting from searching a string (that is, the text returned when a section of a regular expression is enclosed in parentheses) as $1 , $2 , $3 , up to $9 in your code, where $1 holds the text of the first submatch, $2 holds the text of the second submatch, and so on. These values, $1 to $9 , are called backreferences . (They're actually properties of the RegExp object, as discussed at the end of this chapter.)

Here's an example to show how you can use $1 , $2 , $3 , and so on. In this case, I'm matching three words separated by spaces (spaces are matched with the \s code) using the regular expression /(\w+)\s*(\w+)\s*(\w+)/ . Because I've surrounded the match to each of the three words in parentheses, I can refer to those words in code just as $1 , $2 , and $3 , which means that to reverse the order of those words, I could just assemble a replacement string using those matches in reverse order:

(Listing 20-09.html on the web site)

 <HTML>      <HEAD>          <TITLE>The Reverser</TITLE>          <SCRIPT LANGUAGE="JavaScript">              <!--             function reversem()              {  var regexp =/(\w+)\s*(\w+)\s*(\w+)/   document.form1.text1.value = document.form1.text1.value.replace(regexp,  "  ")  }              //-->          </SCRIPT>      </HEAD>      <BODY>          <H1>The Reverser</H1>          <FORM NAME="form1">              Type three words:              <BR>              <INPUT TYPE="TEXT" NAME="text1">              <BR>              <INPUT TYPE="BUTTON" VALUE="Reverse Order" ONCLICK="reversem()">          </FORM>      </BODY>  </HTML>

You can see the results of this code in Figure 20.7. Note that you don't have to use $1 , $2 , $3 , up to $9 to refer to submatches at all of courseyou can use the array returned by the exec or match methods (which can hold many more than nine submatches). However, $1 , $2 , and $3 match the original regular expression syntax in languages such as Perl, and that syntax is available in JavaScript as well.

Figure 20.7. Reversing word order.

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