function escapeRegExpChar(chr, leadingChar, whitespaceChar) {
	    if (leadingChar) {
	      chr = regexpEscapes[chr];
	    } else if (whitespaceChar) {
	      chr = stringEscapes[chr];
	    }
	    return '\\' + chr;
	  }
Used by `_.escapeRegExp` to escape characters for inclusion in compiled regexes.

@private
@param {string} chr The matched character to escape.
@param {string} leadingChar The capture group for a leading character.
@param {string} whitespaceChar The capture group for a whitespace character.
@returns {string} Returns the escaped character.