PERL Beginners - entering regular expressions from the keyboard

Author

entering regular expressions from the keyboard

Christopher Spears

2007-08-21, 4:01 am

Hi!

I'm trying to get back into Perl again by working
through Intermediate Perl. Unfortunately, the Perl
part of my brain has atrophied!

I'm working on the second exercise of the second
chapter. I'm supposed to write a program that asks
the user to type a regular expression. The program
then uses the regular expression to try to find a
match in the directory that I hard coded into the
program. Here is what I have so far:

#!/usr/bin/perl -w
use strict;

print "Enter regular expression: ";

chomp(my $regexp = <STDIN> );
#print $regexp;

opendir(CPPDIR,"/home/io/chris_cpp/") or die "Could
not open directory: $!";
my @allfiles = readdir CPPDIR;
closedir CPPDIR;

foreach $_(@allfiles){
if ($_ =~ \$regexp){
print $_."\n";
}
}

My problem lies with the matching part. I'm not sure
how to use the string that I stored in the $regexp
variable as a regular expression. Any hints?

"I'm the last person to pretend that I'm a radio. I'd rather go out and be a color television set."
-David Bowie

"Who dares wins"
-British military motto

"I generally know what I'm doing."
-Buster Keaton

Jeff Pang

2007-08-21, 4:01 am

-----Original Message-----
>From: Christopher Spears <cspears2002@yahoo.com>
>Sent: Aug 21, 2007 11:28 AM
>To: beginners@perl.org
>Subject: entering regular expressions from the keyboard
>
>Hi!
>
>I'm trying to get back into Perl again by working
>through Intermediate Perl. Unfortunately, the Perl
>part of my brain has atrophied!
>
>I'm working on the second exercise of the second
>chapter. I'm supposed to write a program that asks
>the user to type a regular expression. The program
>then uses the regular expression to try to find a
>match in the directory that I hard coded into the
>program. Here is what I have so far:
>
>#!/usr/bin/perl -w
>use strict;
>
>print "Enter regular expression: ";
>
>chomp(my $regexp = <STDIN> );
>#print $regexp;

$regexp = quotemeta($regexp);

See also perldoc -f quotemeta.

--
Jeff Pang - pangj@earthlink.net
http://home.arcor.de/jeffpang/

Mr. Shawn H. Corey

2007-08-21, 4:01 am

Christopher Spears wrote:
> Hi!
>
> I'm trying to get back into Perl again by working
> through Intermediate Perl. Unfortunately, the Perl
> part of my brain has atrophied!
>
> I'm working on the second exercise of the second
> chapter. I'm supposed to write a program that asks
> the user to type a regular expression. The program
> then uses the regular expression to try to find a
> match in the directory that I hard coded into the
> program. Here is what I have so far:
>
> #!/usr/bin/perl -w
> use strict;
>
> print "Enter regular expression: ";
>
> chomp(my $regexp = <STDIN> );
> #print $regexp;
>

# from here

> opendir(CPPDIR,"/home/io/chris_cpp/") or die "Could
> not open directory: $!";
> my @allfiles = readdir CPPDIR;
> closedir CPPDIR;

# try:
my @allfiles = glob( '*' );

>
> foreach $_(@allfiles){
> if ($_ =~ \$regexp){

# bad regular expression. try:
if( /$regexp/ ){

> print $_."\n";
> }
> }
>
> My problem lies with the matching part. I'm not sure
> how to use the string that I stored in the $regexp
> variable as a regular expression. Any hints?
>
>
>
> "I'm the last person to pretend that I'm a radio. I'd rather go out and be a color television set."
> -David Bowie
>
> "Who dares wins"
> -British military motto

"Of course we'll win; we're British"
- another British military motto

>
> "I generally know what I'm doing."
> -Buster Keaton
>

--
Just my 0.00000002 million dollars worth,
Shawn

"For the things we have to learn before we can do them, we learn by doing them."
Aristotle

Paul Lalli

2007-08-21, 7:59 am

On Aug 20, 11:28 pm, cspears2...@yahoo.com (Christopher Spears) wrote:
> I'm working on the second exercise of the second
> chapter. I'm supposed to write a program that asks
> the user to type a regular expression. The program
> then uses the regular expression to try to find a
> match in the directory that I hard coded into the
> program. Here is what I have so far:
>
> #!/usr/bin/perl -w
> use strict;
>
> print "Enter regular expression: ";
>
> chomp(my $regexp = <STDIN> );
> #print $regexp;
>
> opendir(CPPDIR,"/home/io/chris_cpp/") or die "Could
> not open directory: $!";
> my @allfiles = readdir CPPDIR;
> closedir CPPDIR;
>
> foreach $_(@allfiles){
> if ($_ =~ \$regexp){
> print $_."\n";
> }
>
> }
>
> My problem lies with the matching part. I'm not sure
> how to use the string that I stored in the $regexp
> variable as a regular expression. Any hints?

Shawn and Jeff each gave you half of the answer.

Jeff pointed out that when your pattern is contained in a variable,
you should use quotemeta(). This will backslash any metacharacters
the variable might contain, so that they match themselves rather than
being "special" in the pattern match (so any periods match periods,
rather than "any character", plus signs match plus signs, rather than
meaning "one or more of the previous", etc):
$regexp = quotemeta($regexp)

And Shawn pointed out that the proper syntax for a pattern match is:
$_ =~ /$regexp/

Those two lines should be combined:
$regexp = quotemeta($regexp);
foreach $_(@allfiles){
if ($_ =~ /$regexp/){
print $_."\n";
}
}

Or, instead of calling quotemeta() explicitly, you can use the \Q and
\E escape sequences to do the backquoting within the pattern match
itself:

foreach $_ (@allfiles) {
if ($_ =~ /\Q$regexp\E/) {
print $_ . "\n";
}
}

Also note that an experienced Perl programmer would either eliminate
the $_ whenever it's not needed:
foreach (@allfiles) {
if (/\Q$regexp\E/) {
print "$_\n";
}
}

Or would use a better variable name as the loop iterator:
foreach my $file (@allfiles) {
if ($file =~ /\Q$regexp\E/) {
print "$file\n";
}
}

Hope that helps,
Paul Lalli

Dr.Ruud

2007-08-21, 9:59 pm

Jeff Pang schreef:
> Christopher Spears:

>
> $regexp = quotemeta($regexp);

Since it specifically asks for a regular expression, I would definitely
not do quotemeta().

[color=darkred]

Make that

print qr/$regexp/;

--
Affijn, Ruud

"Gewoon is een tijger."

Jay Savage

2007-08-23, 7:00 pm

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Dr.Ruud

2007-08-24, 4:01 am

Jay Savage schreef:
> Dr.Ruud:
[color=darkred]
>
> Not sure where your headed with this.

My headed? :)

It was an alternative for the commented "debug" line.

> First, OP wants to print the input back to the user.

And I presume that it is more a developer directed print statement.

--
Affijn, Ruud

"Gewoon is een tijger."

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