String Objects and Regular Expressions in JavaScript

The string object has a method called search. This method looks for a match in an available string, and returns the index of the match if a match occurred, or -1 if the match failed. Index counting in a JavaScript string begins from zero. The index obtained is the position of the first character in a matched sub-string. The syntax is:

var returnedIndex = availableString.search(regexp)

Note: If the search fails (no match found) the returnedIndex is -1.

The following script illustrates the case where the search is successful. Try the code.

<script type = "text/javascript">

var availableString = "This is a girl. Another phrase.";

var re = /girl/;

var returnedIndex = availableString.search(re);

alert(returnedIndex);

</script>

The index of the string object obtained here is opposite in nature to the lastIndex of the regexp object, which gives you the position just after the match. The index here gives the position of the first character of the sub-string for the match.

The following script illustrates the case where the search fails. Try the code.

<script type = "text/javascript">

var availableString = "This is a girl. Another phrase.";

var re = /boy/;

var returnedIndex = availableString.search(re);

alert(returnedIndex);

</script>

Literal Text Format or the RegExp Constructor Function

The literal notation provides compilation of the regular expression when the expression is evaluated. Use literal notation when the regular expression will remain constant. For example, if you use literal notation to construct a regular expression used in a loop, the regular expression won't be recompiled on each iteration.

The constructor of the regular expression object, for example, new RegExp("ab+c"), provides runtime compilation of the regular expression. Use the constructor function when you know the regular expression pattern will be changing, or you don't know the pattern and are getting it from another source, such as user input.

RegExps and HTML Window

A separate predefined RegExp object is available in each window; that is, each separate thread of JavaScript execution gets its own RegExp object. Because each script runs to completion without interruption in a thread, this assures that different scripts do not overwrite values of the RegExp object.

We have come to the end of the series. I hope you appreciated it.