Why is my RegExp ignoring start and end of strings?

I made this helper function to find single words, that are not part of bigger expressions

it works fine on any word that is NOT first or last in a开发者_JAVA百科 sentence, why is that?

is there a way to add "" to regexp?

String.prototype.findWord = function(word) {

    var startsWith = /[\[\]\.,-\/#!$%\^&\*;:{}=\-_~()\s]/ ;
    var endsWith = /[^A-Za-z0-9]/ ;

    var wordIndex = this.indexOf(word);

    if (startsWith.test(this.charAt(wordIndex - 1)) &&
        endsWith.test(this.charAt(wordIndex + word.length))) {

        return wordIndex;
    }
    else {return -1;}
}

Also, any improvement suggestions for the function itself are welcome!

UPDATE: example: I want to find the word able in a string, I waht it to work in cases like [able] able, #able1 etc.. but not in cases that it is part of another word like disable, enable etc

---

A different version:

String.prototype.findWord = function(word) {
   return this.search(new RegExp("\\b"+word+"\\b"));
}

Your if will only evaluate to true if endsWith matches after the word. But the last word of a sentence ends with a full stop, which won't match your alphanumeric expression.

---

Did you try word boundary -- \b?

There is also \w which match one word character ([a-zA-Z_]) -- this could help you too (depends on your word definition).

See RegExp docs for more details.

---

If you want your endsWith regexp also matches the empty string, you just need to append |^$ to it:

var endsWith = /[^A-Za-z0-9]|^$/ ;

Anyway, you can easily check if it is the beginning of the text with if (wordIndex == 0), and if it is the end with if (wordIndex + word.length == this.length).

It is also possible to eliminate this issue by operating on a copy of the input string, surrounded with non-alphanumerical characters. For example:

var s = "#" + this + "#"; 
var wordIndex = this.indexOf(word) - 1;

But I'm afraid there is another problems with your function: it would never match "able" in a string like "disable able enable" since the call to indexOf would return 3, then startsWith.test(wordIndex) would return false and the function would exit with -1 without searching further.

So you could try:

String.prototype.findWord = function (word) {

   var startsWith = "[\\[\\]\\.,-\\/#!$%\\^&\*;:{}=\\-_~()\\s]";
   var endsWith =   "[^A-Za-z0-9]";

   var wordIndex = ("#"+this+"#").search(new RegExp(startsWith + word + endsWith)) - 1;

   if (wordIndex == -1) { return -1; }
   return wordIndex;
}