How do I change the perl regex for code formatting? - c

Question

I am trying to understand the below code. But I am not getting it.
Basically, the below code currently checks for if condition in a c or cpp file.
if ($perl_version_ok &&
$line =~ /^\+(.*)\b($Constant|[A-Z_][A-Z0-9_]*)\s*($Compare)\s*($LvalOrFunc)/) {
# Throw error
}
where Constant is any macro or any constant value; LvalOrFunc is any variable or function call; Compare is the operations like !=, == ,&& , etc
The if checks for codes like this if(CONST_VALUE == x), where CONST_VALUE is some macros. In this case its true and goes inside if condition.
But I want to check for the opposite if(x == CONST_VALUE ), and then throw error.
Please help in understanding this piece of line and how to achieve the desired result.
Note:
The code is from linux kernel dir, available here: https://github.com/torvalds/linux/blob/master/scripts/checkpatch.pl
Line number of the code: 5483

Answer 1

The code doesn't check for if(CONST_VALUE == x). As shown in the comments above the line in the source code
# comparisons with a constant or upper case identifier on the left
# avoid cases like "foo + BAR < baz"
# only fix matches surrounded by parentheses to avoid incorrect
# conversions like "FOO < baz() + 5" being "misfixed" to "baz() > FOO + 5"
it checks for a plus sign followed by a CONSTANT_VALUE == x. The \+ in the regex matches a plus sign.
$line =~ /^\+(.*)\b($Constant|[A-Z_][A-Z0-9_]*)\s*($Compare)\s*($LvalOrFunc)/
^ ^ ^ ^ ^
| | | | |
binding | | | word
operator | | | boundary
start | |
of string | |
plus |
anything
Reverting the compared values should be easy:
($LvalOrFunc)\s*($Compare)\s*($Constant|[A-Z_][A-Z0-9_]*)