How to use an user input as a regex?

Question

I have a simple program where the user can enter a string. After this the user can enter a regex. I need the string to be compared against this regex.

The following code do not work - the regex always fails.

And I know that its maybe because I am comparing a string with a string and not a string with a regex.

But how would you do this?

while(1){
    print "Enter a string: ";
    $input = <>;
    print "\nEnter a regex and see if it matches the string: ";
    $regex = <>;

    if($input =~ $regex){
        print "\nThe regex $regex matched the string $input\n\n";
    }
}

daxim · Accepted Answer · 2012-04-27 11:16:46Z

2

Use lexical variables instead of global ones.
You should remember that strings read by <> usually contain newlines, so it might be necessary to remove the newlines with chomp, like this:
```
chomp(my $input = <STDIN>);
chomp(my $regex = <STDIN>);
```
You might want to interpret regex special characters taken from the user literally, so that ^ will match a literal circumflex, not the beginning of the string, for example. If so, use the \Q escape sequence:
```
if ($input =~ /\Q$regex\E/) { ... }
```
Don't forget to read the Perl FAQ in your journey through Perl. It might have all the answers before you even begin to specify the question: How do I match a regular expression that's in a variable?

edited Apr 27, 2012 at 11:16

daxim

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answered Apr 27, 2012 at 10:15

raina77ow

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2 Comments

Diemauerdk Over a year ago

Thx for the fast reply :) But i still cant get this to work :(. The string wont match the regex.

raina77ow Over a year ago

Could you provide that string and regex?

geekosaur · Accepted Answer · 2012-04-27 10:06:13Z

1

You need to use a //, m//, or s/// — but you can specify a variable as the pattern.

if ($input =~ /$regex/) {
  print "match found\n";
}

answered Apr 27, 2012 at 10:06

geekosaur

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3 Comments

geekosaur Over a year ago

@NikhilJain, I was talking about the syntax required in general: if you use =~ or !~ then you need to apply it to a matching type of operator.

Nikhil Jain Over a year ago

yes, actually I thought the same thing that you are providing the options in general that's why I deleted my comment.

Diemauerdk Over a year ago

your solution works. As regex i entered /something_to_match/ where i just should type something_to_match without /.

skywalker · Accepted Answer · 2012-04-27 10:09:41Z

0

I think you need to chomp input and regex variables. and correct the expression to match regex

chomp( $input );
chomp( $regex );
if($input =~ /$regex/){
    print "\nThe regex $regex matched the string $input\n\n";
}

answered Apr 27, 2012 at 10:09

skywalker

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