in reply to CGI Help

also I am sort of confused by this regex and I think I am confused because we have not used grep before

It probably selects only filenames which end in jpg or gif. Tip #9 from the Basic debugging checklist (YAPE::Regex::Explain) 

The regular expression:

(?i-msx:\.(?:gif|jpg)$)

matches as follows:
  
NODE                     EXPLANATION
----------------------------------------------------------------------
(?i-msx:                 group, but do not capture (case-insensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  \.                       '.'
----------------------------------------------------------------------
  (?:                      group, but do not capture:
----------------------------------------------------------------------
    gif                      'gif'
----------------------------------------------------------------------
   |                        OR
----------------------------------------------------------------------
    jpg                      'jpg'
----------------------------------------------------------------------
  )                        end of grouping
----------------------------------------------------------------------
  $                        before an optional \n, and the end of the
                           string
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------

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Re^2: CGI Help
by phizymonk (Novice) on Feb 10, 2016 at 19:20 UTC

    is there a certain situation you shoud use grep in and not to use grep in?

[reply]
      grep in Perl is essentially a simple method for filtering a list. I tend to recommend avoiding it while you are still familiarizing yourself with Perl's constructs.

      grep allows you to accomplish something like: 

      my @second;
for my $element (@first) {
    push @second, $element if condition($element);
}

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      in a concise syntax. Simple filtering by a regular expression is a classic use case. If you have side effects or complex tests, then grep may be a bad choice.

      #11929 First ask yourself `How would I do this without a computer?' Then have the computer do it the same way.

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