perl how to extract #define'd number from C header file - c

Question

I am looking for a Perl snippet to print the number 100, contained in a C header, to the terminal.
#ifndef VERSIONS_H
#define VERSIONS_H
#define __SOFTWARE_REVISION__ 100
#endif
I am able to extract the #define line, but the number.
#!perl -w
use warnings;
use strict;
my $line;
my $file = shift #ARGV;
open my $fh, "<", $file or die $!;
while (<$fh>) {
print if /__SOFTWARE_REVISION__/;
}
close ($fh);

Answer 1

The regex matches for any line that contains that literal phrase, not only where it is defined. You need to specify the line far more precisely, and to capture the needed number.
The code also proceeds to loop through the file needlessly.
With a few more adjustments
use warnings;
use strict;
use feature qw(say);
my $file = shift #ARGV;
die "Usage: $0 filename\n" if not $file or not -f $file;
open my $fh, '<', $file or die "Can't open $file: $!";
while (<$fh>) {
if (/^\s*#\s*define\s+__SOFTWARE_REVISION__\s+([0-9]+)/) {
say "Software revision: $1";
last;
}
}
close $fh;
If the revision can be other than integer replace [0-9]+ with a the pattern [0-9.]+ that allows a period, or [0-9.a-z] if (low-case) letters are allowed in versioning ... or simply \S+.
I'd suggest to work through the tutorial perlretut with reference perlre on hand.
I assume that this need comes up in a Perl program (and not that Perl is the chosen tool for this).
Still, note that there are other, more systemic, ways to retrieve this information using the compiler (see a comment on gcc by rici for example).
While it is better to use Perl in a Perl script rather than go out to the system, this case may be one of exceptions: it may be better to use an external command rather than parse source files, a task well known to be treacherous.

Answer 2

Here's an approach using regex to capture the version number if present in a line. Note that parenthesis in the regex delimit the capturing group and the variable $1 is used to access the capture.
#!perl -w
use warnings;
use strict;
my $file = shift #ARGV;
open my $fh, "<", $file or die $!;
while (<$fh>) {
print "$1\n" if /^#define +__SOFTWARE_REVISION__ +(\d+)$/;
}
close $fh;
Output:
100
The regex is more selective than yours, which matches any instance of __SOFTWARE_REVISION__ in a line. This regex matches #define at the beginning of line, followed by one or more spaces, followed by __SOFTWARE_REVISION__, followed by one or more spaces, followed by a one or more digit capturing group at the end of the line.