Why does this non-greedy match not work?

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

Suppose I have the following in a text file "fbb_test":

/foo/bar/baz
[download]

Now suppose I have the following script:

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

my $file = "/tmp/fbb_test";

open(FILE, $file) || die("ERROR: Unable to open $file for read, exitin
+g...\n");

while (<FILE>)
{
    chomp($_);

    if ($_ =~ /\/(.+?)$/)
    {
        printf("captured $1\n");
    }
}

close(FILE);
[download]

The script is returning "captured foo/bar/baz". Given that I specified the non-greedy operator (?), I would have expected the result to be "baz", as the non-greedy operator would have matched as few characters as possible between a forward slash and the end of the line. What am I missing here?

Comment on Why does this non-greedy match not work? Select or Download Code

Replies are listed 'Best First'.
Re: Why does this non-greedy match not work? by Fletch (Bishop) on Jun 29, 2020 at 17:41 UTC
Making the `.+` non-greedy doesn't change that the dot is perfectly willing to match any `/` in the inputs (since a dot just matches any (non-newline, by default) character). It's still going to start at the left-most "/" that it sees (the first character) and then start gobbling up one or more things-which-match-dot (which includes all the subsequent letters and "/"s) until it hits the end of the line. If you'd explicitly asked for things-which-are-not-"/" using `[^/]+` instead you'd have had more luck (e.g. `m{/ ([^/]+) $}x`). Of course this could just be a slight XY problem and you really want File::Basename or Path::Tiny instead . . . The cake is a lie. The cake is a lie. The cake is a lie.	[reply] [d/l] [select]
Re^2: Why does this non-greedy match not work? (updated) by haukex (Archbishop) on Jun 29, 2020 at 19:07 UTC
I agree entirely, and just wanted to add: a common misconception (one that I was guilty of myself sometimes, before being enlightened about it) is that a regex that doesn't start with `^` but ends with `$` somehow changes the behavior of the regex engine to start looking at the end of the string - this isn't how it works, the regex engine always matches from left to right and stops at the first match it finds. A regex of `"/foo/bar/baz" =~ m{/[^/]+$}` will still cause the regex engine to attempt to match `/foo` and `/bar` before settling on `/baz` - you can see this in action at this link (JavaScript and a modern browser required) when you click the `use re "debug";` button. Update: Corrected `=` to `=~`, thanks hippo!	[reply] [d/l] [select]
Re: Why does this non-greedy match not work? by roho (Bishop) on Jun 30, 2020 at 10:22 UTC
Just a few code nits. Use lexical file handles and the three-argument form of open. The "\|\|" operator in the open statement works because of the presence of parens, but the "or" operator is safer because it has a lower precedence, in case parens are not present. "It's not how hard you work, it's how much you get done."	[reply]


Do you know where your variables are?
	PerlMonks