Question

I have a problem replace certain words started with #. I have the following code

var x="#google",
eval("var pattern = /" + '\\b' + x + '\\b');
txt.replace(pattern,"MyNewWord");

when I use the following code it works fine

var x="google",
eval("var pattern = /" + '\\b' + x + '\\b');
txt.replace(pattern,"MyNewWord");

it works fine

any suggestion how to make the first part of code working

ps. I use eval because x will be a user input.

Answer 1

The problem is that \b represents a boundary between a “word” character (letter, digit, or underscore) and a “non-word” character (anything else). # is a non-word character, so \b# means “a # that is preceded by a word character” — which is not at all what you want. If anything, you want something more like \B#; \B is a non-boundary, so \B# means “a # that is not preceded by a word character”.

I’m guessing that you want your words to be separated by whitespace, instead of by a programming-language concept of what makes something a “word” character or a “non-word” character; for that, you could write:

var x = '#google';    // or 'google'
var pattern = new RegExp('(^|\\s)' + x);
var result = txt.replace(pattern, '$1' + 'MyNewWord');

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Edited to add: If x is really supposed to be a literal string, not a regex at all, then you should “quote” all of the special characters in it, with a backslash. You can do that by writing this:

var x = '#google';  // or 'google' or '$google' or whatever
var quotedX = x.replace(/[^\w\s]/g, '\\$&');
var pattern = new RegExp('(^|\\s)' + quotedX);
var result = txt.replace(pattern, '$1' + 'MyNewWord');