Matching Regex in Perl - regex

Question

my program contains ascii.txt to match patterns from it.
my program is to implement sed command, just to try write perl code because I am studying perl.
#!/usr/bin/perl
# sed command implementation
use strict;
use warnings;
use subs qw(read_STDIN read_FILE usage);
use IO::File;
use constant {
SEARCH_PRINT => 0,
};
our $proj_name = $0;
main(#ARGV);
sub main
{
if(scalar #_ == 2) {
read_FILE #_;
}
else {
usage
}
}
sub read_FILE {
my ($sed_script, $file_name) = #_;
my $parsed_val = parse_sed_script($sed_script);
if( $parsed_val == SEARCH_PRINT ) {
search_print_lines($sed_script, $file_name);
}
}
sub parse_sed_script {
my $command = shift or return;
if($command =~ /^\/([^\/].)*\/$/) {
return SEARCH_PRINT;
}
}
sub search_print_lines {
my ($script, $file) = #_;
my $fh = IO::File->new($file, "r") or error("no file found $file");
while( $_ = $fh->getline ) {
print if $_ =~ $script
}
}
sub usage {
message("Usage: $proj_name sed-script [file]")
}
sub error
{
my $e = shift || 'unkown error';
print("$0: $e\n");
exit 0;
}
When I execute from the shell: sed.pl /Test/ ascii.txt
I found that print if $_ =~ $script, doesn't execute because of the REGEX is stored in scalar variable
the ascii.txt contains.
Test 1
REGEX TEST
When I use print $script in search_print_lines subroutine it prints the regex sent by the user

Answer 1

When you pass something in on the command line and use it in your script, the entire literal text is used. So if you pass in /Test/, it will see those slashes as literals, so the "real" regular expression it's looking at is something like \/Test\/ (escaping the slashes, because now it's looking for them. Try passing in the regex without the // surrounding it.
If your goal is to allow the // to show that it's a regular expression, I would remove them when the program starts.
One more edit: If you want to be able to pass in flags, you'd need to eval the input somehow.
$script = '/Test/i';
eval { "\$regex = $script" };
and then
"REGEX TEST" =~ $regex
should return true. Doing an eval like this is highly insecure, though.
edit: what happens in eval is that whatever's in the block is executed. So what happens in the eval above is that you're dynamically creating a regular expression and setting it to a variable. That allows you to use regular expression flags like i without having to do any special parsing of the command-line input. When the eval is executed, it will be as if you had typed in $regex = /Test/i. Then you can compare your text to $regex and it will work. I thought about this because your example would not work unless you had the i flag set to make the comparison case-insensitive.

Answer 2

You didn't remove the slashes from $sed_script variable. After I modified your read_FILE function, it started to work:
sub read_FILE {
my ($sed_script, $file_name) = #_;
my $parsed_val = parse_sed_script($sed_script);
if( $parsed_val == SEARCH_PRINT ) {
$sed_script =~ s/^\/(.*)\/$/$1/;
#you can also parse the regexp
#$sed_script = qr/$sed_script/;
search_print_lines($sed_script, $file_name);
}
}