Javascript String Replace Weirdness -- $$$$ Gets Collapsed To $$ -- What's The Reason Behind This Result?

At work, I was encountering a problem where users of our application were receiving messages featuring an invalid unicode character (0xffff), which according to the standard, shoul

Solution 1:

That's because $ in the replace string has special meaning (group expansion). Have a look at this example:

alert("foo".replace(/(.*)/, "a$1b"));

That's why $$ is interpreted as $, for the case where you would need to actually replace something by $1 (literally, without group expansion):

alert("foo".replace(/(.*)/, "a$$1b"));

See also https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/String/replace#Specifying_a_string_as_a_parameter.

Solution 2:

The $ sign is a special character in the replacement argument to denote sub-matches from parentheses in the regex pattern ($1, $2, etc.). So to get a $ you have to "escape" it by typing $$. Which is what you did twice.

Solution 3:

The $ in a replace string is used to signal the use of the match groups $1, $2 etc, si to put a $ into the replace string you need to use two of them.