Question

I am refering to code from Case insensitive string replacement in JavaScript?:

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

What does \\$& mean?

I think \\ escapes the \ character. Then $&, I thought it should be $1 to match the 1st match? tho $1 does not work right

Answer 1

$& represents the entire (sub)string matched by the regex, regardless of capture groups. The replacement result you get is each match in your string being escaped by a literal backslash (represented by \\). Since the regex used here consists only of a character class, “each match” refers to each metacharacter listed in the character class that is matched.

For example, the regex string [abc] will be replaced with \[abc\]:

• [ is matched as it occurs in the character class. Represented by $&, replaced with \[

• a, b and c are not metacharacters in the character class, so they’re ignored

• ] is matched as it occurs in the character class. Represented by $&, replaced with \]