Perl conditional regex extraction

Question

This conditional must match either telco_imac_city or telco_hier_city. When it succeeds I need to extract up to the second underscore of the value that was matched.

I can make it work with this code

if ( ($value =~ /(telco_imac_)city/) || ($value =~ /(telco_hier_)city/) ) {
    print "value is: \"$1\"\n";
}

But if possible I would rather use a single regex like this

$value = $ARGV[0];
if ( $value =~ /(telco_imac_)city|(telco_hier_)city/ ) {
    print "value is: \"$1\"\n";
}

But if I pass the value telco_hier_city I get this output on testing the second value

Use of uninitialized value $1 in concatenation (.) or string at ./test.pl line 19.
value is: ""

What am I doing wrong?

fugu · Accepted Answer · 2014-02-18 21:38:16Z

1

while (<$input>){
    chomp;
    print "$1\n" if /(telco_hier|telco_imac)_city/;
}

answered Feb 18, 2014 at 21:38

fugu

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1 Comment

admdrew Over a year ago

OP wants to return "only up to the 2nd underscore of the value that was matched" (ie, telco_hier_ or telco_imac_).

Community · Accepted Answer · 2017-05-23 12:10:54Z

1

Perl capture groups are numbered based on the matches in a single statement. Your input, telco_hier_city, matches the second capture of that single regex (/(telco_imac_)city|(telco_hier_)city/), meaning you'd need to use $2:

my $value = $ARGV[0];
if ( $value =~ /(telco_imac_)city|(telco_hier_)city/ ) {
    print "value is: \"$2\"\n";
}

Output:

$> ./conditionalIfRegex.pl telco_hier_city
value is: "telco_hier_"

Because there was no match in your first capture group ((telco_imac_)), $1 is uninitialized, as expected.

To fix your original code, use FlyingFrog's regex:

my $value = $ARGV[0];
if ( $value =~ /(telco_hier_|telco_imac_)city/ ) {
    print "value is: \"$1\"\n";
}

Output:

$> ./conditionalIfRegex.pl telco_hier_city
value is: "telco_hier_"

$> ./conditionalIfRegex.pl telco_imac_city
value is: "telco_imac_"

edited May 23, 2017 at 12:10

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answered Feb 18, 2014 at 21:38

admdrew

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2 Comments

fugu Over a year ago

What happens when $value is telco_imac_ ?

admdrew Over a year ago

@FlyingFrog Edited with (a slightly amended version of) your regex, thanks! The original part of the answer is helpful for OP to understand why the uninitialized warning was occurring.

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