Javascript Regex Word Boundary with optional non-w

Question

I am looking to find a keyword match in a string. I am trying to use word boundary, but this may not be the best case for that solution. The keyword could be any word, and could be preceded with a non-word character. The string could be any string at all and could include all three of these words in the array, but I should only match on the keyword:

['hello', '#hello', '@hello'];

Here is my code, which includes an attempt found in post:

let userStr = 'why hello there, or should I say #hello there?';

let keyword = '#hello';

let re = new RegExp(`/(#\b${userStr})\b/`);

re.exec(keyword);

• This would be great if the string always started with #, but it does not.
• I then tried this /(#?\b${userStr})\b/, but if the string does start with #, it tries to match ##hello.
• The matchThis str could be any of the 3 examples in the array, and the userStr may contain several variations of the matchThis but only one will be exact

Answer 1

You need to account for 3 things here:

• The main point is that a \b word boundary is a context-dependent construct, and if your input is not always alphanumeric-only, you need unambiguous word boundaries
• You need to double escape special chars inside constructor RegExp notation
• As you pass a variable to a regex, you need to make sure all special chars are properly escaped.

Use

let userStr = 'why hello there, or should I say #hello there?';
let keyword = '#hello';
let re_pattern = `(?:^|\\W)(${keyword.replace(/[-\/\\^$*+?.()|[\]{}]/g, '\\$&')})(?!\\w)`;
let res = [], m;

// To find a single (first) match
console.log((m=new RegExp(re_pattern).exec(userStr)) ? m[1] : "");

// To find multiple matches:
let rx = new RegExp(re_pattern, "g");
while (m=rx.exec(userStr)) {
    res.push(m[1]);
}
console.log(res);

Pattern description

• (?:^|\\W) - a non-capturing string matching the start of string or any non-word char
• (${keyword.replace(/[-\/\\^$*+?.()|[\]{}]/g, '\\$&')}) - Group 1: a keyword value with escaped special chars
• (?!\\w) - a negative lookahead that fails the match if there is a word char immediately to the right of the current location.

Answer 2

Check whether the keyword already begins with a special character. If it does, don't include it in the regular expression.

var re;
if ("#@".indexOf(keyword[0]) == -1) {
    re = new RegExp(`[@#]?\b${keyword}\b`);
} else {
    re = new RegExp(`\b${keyword}\b`);
}