Reading this discussion, I didn't understand what this means:
$1X$2
Simply X?
And about this other code:
str = str.replace(/(<script.*?>)(.*)(?=<\/script>)/gi, function(x,y,z)
{return y+z.replace(/a/gi,'Z')})
Here is what I didn't understand:
?= function(x,y,z) works? i.e. from where it take x, y and z?I'll be very grateful if someone could explain in a clear way this code.
The $1 and $2 are referencing the captured sub-expressions (which are delimited by parentheses ()) from the regular expression in the previous argument to .replace().
The ?= is a positive lookahead. http://www.regular-expressions.info/lookaround.html
The function(x, y, z) is an anonymous function expression that does a similar thing to referencing $1 and $2, but it stores them in variables instead.
The string "$1X$2" used as the second argument to a .replace() call includes references to groups from the regular expression first argument. Groups — portions of the regular expression in parentheses — collect the characters they match for use later. The substrings "$1" and "$2" mean, "whatever was matched by the group 1" (or group 2).
When the second argument to a call to .replace() is a function, JavaScript passes the match information to the function as arguments. The first argument is always the entire match. The subsequent arguments are the matched groups.
Thus:
alert("hello world".replace(/(ell)(o w)/, "$2$1");
// alerts "ho wellorld"
This is all about capturing groups. ( and ) capture everything between, so you can later access substrings of your match.
$1 refers to the first captured group, $2 to the second one (so first and second pair of parentheses, respectively). ($0 would refer to the whole match.)
The variant with the callback function does the same, but here the variables x, y and z are filled with the captured groups (corresponding to $0, $1 and $2 respectively).
Finally ?= opens a lookahead. That asserts that this possition in the match is followed by <\/script> without actually advancing the regex engine in the matched string and also without including this part in the match.
