Preg_replace for encoding - php

Question

I am really bad in regex expression,
there is below code i saw in my existing application in php but not really understanding what it is doing.
$content = preg_replace('/(<?xml[^>]*)encoding=["\']([^>"\']*)?["\']([^>]*?>)/', '$1encoding="' . $encoding . '"$3', $content);
return $content;
If any expert can share some info that would be really great.
Update
Trying to run this but not working is there anything i am missing here..
$encoding = 'UTF-32';
$content = '<?xml version="1.0" encoding="UTF-8" ?><rss version="2.0" xmlns:media="http://search.yahoo.com/mrss/" xmlns:atom="http://www.w3.org/2005/Atom">
<channel></channel>';
echo preg_replace('/(<?xml[^>]*)encoding=["\']([^>"\']*)?["\']([^>]*?>)/', '$1encoding="' . $encoding . '"$3', $content);
update
It may sound stupid.. but it was working... as it is XML tag so for that i need to do display source in chrome and it will display it...
Thank you everyone.. specially M42
regards,
Mona

Answer 1

It will change the encoding present in the file by a new one contained in the variable $encoding
Example:
<?xml blah blah encode="XXX" blah blah?>
will be changed to (assuming $encoding='UTF-8';):
<?xml blah blah encode="UTF-8" blah blah?>
$1 contains: <xml blah blah
$3 contains: blah blah>
The result is the concatenation of $1, encode="UTF-8" and $3.
Regex explanation:
The regular expression:
(<?xml[^>]*)encoding=["\']([^>"\']*)?["\']([^>]*?>)
matches as follows:
NODE EXPLANATION
----------------------------------------------------------------------
( group and capture to \1:
----------------------------------------------------------------------
<? '<' (optional (matching the most amount
possible))
----------------------------------------------------------------------
xml 'xml'
----------------------------------------------------------------------
[^>]* any character except: '>' (0 or more
times (matching the most amount
possible))
----------------------------------------------------------------------
) end of \1
----------------------------------------------------------------------
encoding= 'encoding='
----------------------------------------------------------------------
["\'] any character of: '"', '\''
----------------------------------------------------------------------
( group and capture to \2 (optional
(matching the most amount possible)):
----------------------------------------------------------------------
[^>"\']* any character except: '>', '"', '\'' (0
or more times (matching the most amount
possible))
----------------------------------------------------------------------
)? end of \2 (NOTE: because you are using a
quantifier on this capture, only the LAST
repetition of the captured pattern will be
stored in \2)
----------------------------------------------------------------------
["\'] any character of: '"', '\''
----------------------------------------------------------------------
( group and capture to \3:
----------------------------------------------------------------------
[^>]*? any character except: '>' (0 or more
times (matching the least amount
possible))
----------------------------------------------------------------------
> '>'
----------------------------------------------------------------------
) end of \3
----------------------------------------------------------------------