perl: using 'substr' in regex

ananthbv · 11-02-2004, 01:31 AM

hi guys,
i wrote the following program in perl.

Code:

open(TEMP, "temp_file"); # this file is nothing but a list of filenames, one name in each line.

@arr = <TEMP>;

$temp = "@arr";

$line = "M1BRMA00"; # this line exists in "temp_file"

if ($temp =~ /substr($line, 0, 5)/)
{
    print "found";
}
else
{
    print "not found";
}

surprisingly(to me, that is) it printed "not found".

but when i assign the 'substr' part to another string, as in this prog,

Code:

open(TEMP, "temp_file"); 

@arr = <TEMP>;

$temp = "@arr";

$line = "M1BRMA00"; # this line exists in "temp_file"
$tempstr = substr($line, 0, 5);

if ($temp =~ /$tempstr/)
{
    print "found";
}
else
{
    print "not found";
}

it prints "found" which is correct. why? since 'substr' returns a subset of the target string, what is the difference between the two programs?

thanks for any help.

secesh · 11-02-2004, 01:36 AM

the difference is in the regular expression -- check your syntax on how to construct one -- you cannot just throw code in a regex all willy-nilly like and expect it to work....

--Regular Expressions have their own syntax, which differs from that of perl!

secesh · 11-02-2004, 01:51 AM

more:
i'm not really looking to trace through your code there, but why aren't you using a 'foreach'... seems to me you're lucky to be getting a found -- $temp = "@arr"??

maybe i just haven't written any perl in a while, but i would make it something like:

Code:

#!/usr/bin/perl
###########
## Moray ##
###########
open(TEMP, "temp_file"); 
my @lines_of_temp = <TEMP>;
close(TEMP);
$find_me = "M1BRMA00";

foreach (@lines_of_temp){
    if(m/$find_me/){
        # found
    }else{
        print "not ";
    }
    print "found\n";
}

--yup, a quick modification and run, and that there is some good code!

[edit] add a counter to that, or something to not print every line... i ran this on a 9-line file, and it printed an accurate 'found' on line 8 of the result... there will be one line printed for every line in your file without any modification...[/edit]

ananthbv · 11-02-2004, 03:08 AM

thanks for ur replies secesh. yeah, the '(' and ')' is the problem, right? because it is part of regex syntax. stupid of me to ask the question in the first place. but how do i include a function in a regex? or pls tell me where i can read up on it.
thanks.

secesh · 11-02-2004, 06:52 PM

regexes can be very complex -- i have never really gone that deep with them... start with google - there's tons of documention on them... sorry i can't help more.

Scorpions4ever · 11-03-2004, 01:58 AM

Something like this ought to do the trick.

Code:

#!/usr/bin/perl -w

$line = "MBR100456";
$tmp = "MBR100678";
if ($tmp =~ /(?{substr($line, 0, 5);})/) {
    print "Found\n";
} else {
    print "Not found\n";
}