Exact pattern match inside index()

jayarkay has asked for the wisdom of the Perl Monks concerning the following question:

Hello,

I am trying to use the index() function and I want to find the position of a word inside a string, only when it is an exact match. For example:

My string is STRING="CATALOG SCATTER CAT CATHARSIS"

And my search string is KEY=CAT

I want to say something like index($STRING, $KEY) and get the position of CAT, and not CATALOG. How do I accomplish this? The documentation says "The index function searches for one string within another, but without the wildcard-like behavior of a full regular-expression pattern match", which makes me think that it may not be that straight-forward, but my perl skills are limited :). Is it possible to do what I am trying to do?

Hopefully, I was able to articulate my question well. Thanks in advance for your help!

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Re: Exact pattern match inside index() by kennethk (Abbot) on Oct 15, 2012 at 20:27 UTC
index does a check for literal characters. By specifying you want `CAT` but not `CATALOG`, you are adding criteria that cannot be expressed using literal characters only. You could get closer by using `$KEY = " CAT "`, since your string is whitespace delimited, but this would fail if `CAT` is either your first or last entry. The only way to implement this robustly would be to use a tool that can check for non-literal character criteria, e.g. regular expressions. For example, you could use the word anchor `\b` to specify you want word boundaries on either side of `CAT` -- see Using character classes. You can then use `$-[0]` (see @-) to learn the position of that match. So you might say `my $string = "CATALOG SCATTER CAT CATHARSIS"; my $key = 'CAT'; $string =~ /\b\Q$key\E\b/; print "$key found at $-[0]\n";` [download] #11929 First ask yourself `How would I do this without a computer?' Then have the computer do it the same way.	[reply] [d/l] [select]
Re: Exact pattern match inside index() by toolic (Bishop) on Oct 15, 2012 at 20:21 UTC
You could use a regex with pos: `use warnings; use strict; my $s = 'CATALOG SCATTER CAT CATHARSIS'; my $k = 'CAT'; if ($s =~ /\b$k\b/g) { print "match started at position ", (pos($s) - length($k)), "\n"; } __END__ match started at position 16` [download] UPDATE: kennethk's `@-` is much cleaner.	[reply] [d/l] [select]
Re: Exact pattern match inside index() by Anonymous Monk on Oct 15, 2012 at 20:20 UTC
You are matching for `\bCAT\b` with a regular expression. From `perldoc perlre`: A word boundary ("\b") is a spot between two characters that has a "\w" on one side of it and a "\W" on the other side of it (in either order), counting the imaginary characters off the beginning and end of the string as matching a "\W". (Within character classes "\b" represents backspace rather than a word boundary, just as it normally does in any double‐quoted string.) \w Match a "word" character (alphanumeric plus "_") \W Match a non-"word" character	[reply]
Re: Exact pattern match inside index() by jayarkay (Initiate) on Oct 17, 2012 at 15:48 UTC
Thank you all for your responses. I apologize for the delay in responding. Both toolic and Kennethk's suggestions worked. And as toolic added in the update, kennethk's option of using `$-[0]` seemed cleaner. Thanks also for the references.	[reply] [d/l]