Understanding Perl match and index functions

Question

I've inherited the following Perl code, and I'm having difficulty understanding what exactly the index and match functions are doing here:

my $url = $ENV{'REQUEST_URI'};
my $loc = $url;
$loc =~ s/\/parks\///i;
my $page = substr($loc, 0, index $loc, "_");

I know that index(str, char) returns the index of a particular character, so what function does index $loc serve? Is it simply returning the length?

I'm also confused by the line $loc =~ s/\/parks\///i; which matches the phrase "/parks/" in the url. I don't see the purpose of it, because whether it returns true or false, isn't $loc still a string containing the url? What should I expect to be contained in $page?

I am very new to Perl so I appreciate that there will be some nuances that I haven't yet grasped.

Dave Cross · Accepted Answer · 2011-10-19 23:03:00Z

3

Perl's slightly cavalier approach to parenthesis might be confusing you a little here. The calls to substr and index might be more clearly written as:

my $page = substr($loc, 0, index($loc, "_"));

It's looking for the first occurrence of '_' in $loc and truncating the variable at that point.

Also, the substitution operator would be easier to understand if you used alternative delimiters.

$loc =~ s|/parks/||i;

It means "look for the first occurrence of "/parks/" in $loc and replace it with an empty string (i.e. remove it).

edited Oct 19, 2011 at 23:03

answered Oct 19, 2011 at 12:46

Dave Cross

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Comments

Toto · Accepted Answer · 2011-10-19 12:40:17Z

1

$loc =~ s/\/parks\///i;

This will remove /parks/ from the string $loc

my $page = substr($loc, 0, index $loc, "_");

can be written as:

# retrieve the position of the first "_" in the string $loc 
my $index = index $loc, "_";
# keep the begining of the string from position 0 to position $index
my $page = substr($loc, 0, $index);

answered Oct 19, 2011 at 12:40

Toto

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1 Comment

runrig Over a year ago

Not quite. $index becomes the length argument in substr, so you get everything up to the character before the first "_"

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