I'm using startswith reg exp in Javascript

if ((words).match("^" + string)) 

but if I enter the characters like , ] [ \ /, Javascript throws an exception. Any idea?

I'm using startswith reg exp in Javascript

if ((words).match("^" + string)) 

but if I enter the characters like , ] [ \ /, Javascript throws an exception. Any idea?

• javascript
• regex

4 Answers 4

If you're matching using a regular expression you must make sure you pass a valid Regular Expression to match(). Check the list of special characters to make sure you don't pass an invalid regular expression. The following characters should always be escaped (place a \ before it): [\^$.|?*+()

A better solution would be to use substr() like this:

if( str === words.substr( 0, str.length ) ) {
   // match
}

or a solution using indexOf is a (which looks a bit cleaner):

if( 0 === words.indexOf( str ) ) {
   // match
}

next you can add a startsWith() method to the string prototype that includes any of the above two solutions to make usage more readable:

String.prototype.startsWith = function(str) {
    return ( str === this.substr( 0, str.length ) );
}

When added to the prototype you can use it like this:

words.startsWith( "word" );

One could also use indexOf to determine if the string begins with a fixed value:

str.indexOf(prefix) === 0

If you want to check if a string starts with a fixed value, you could also use substr:

words.substr(0, string.length) === string

If you really want to use regex you have to escape special characters in your string. PHP has a function for it but I don't know any for JavaScript. Try using following function that I found from [Snipplr][1]

function escapeRegEx(str)
{
   var specials = new RegExp("[.*+?|()\\[\\]{}\\\\]", "g"); // .*+?|()[]{}\
   return str.replace(specials, "\\$&");
}

and use as

var mystring="Some text";
mystring=escapeRegEx(mystring);

If you only need to find strings starting with another string try following

String.prototype.startsWith=function(string) {
   return this.indexOf(string) === 0;
}

and use as

var mystring="Some text";
alert(mystring.startsWith("Some"));