Beginner confused with string matching

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

Hi, I recently started learning perl because I need to write some code to modify some netlists. For now, I'm trying to figure out a script written by someone else. I mostly understand the logic, I'm stuck at a point where he uses a comparison.

if ($val =~ /^[ ]+Pin:[ ]*([^\.]*)\.([^ ]*)[ ]*out/) {
            $cellname = $mod->find_cell($1);
            $pin_connected = $2;
          }
[download]

How would I go about deciphering the thing after $val? Also, the verilog-perl module is used here. I'm struggling to figure out what the "$1" does in $mod->find_cell($1) Is it something related to perl? I could not find anything in the verilog perl module documentation. Thanks.

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Re: Beginner confused with string matching by hippo (Archbishop) on Apr 09, 2015 at 15:08 UTC
Welcome to the wonderful world of Perl. The thing after the $val is called a "Regular Expression" and is, arguably, one of Perl's finest features. You can read all about it at perlre and the tutorial at perlretut The $1 (and $2) fall out from the use of the regular expression and specifically the capture groups within it which are delimited by the parentheses, eg. whatever is matched by `([^\.]*)` is stored in $1, etc. They are not related to anything specific in the verilog module. The documentation for Perl is also a strong point and I encourage you to become familiar with it and how to use it at the earliest opportunity. You can read it online or from your local machine with the handy `perldoc` command. If you are still stuck parsing the regular expression (regex) after this, then by all means ask further.	[reply] [d/l] [select]
Re: Beginner confused with string matching by Anonymous Monk on Apr 09, 2015 at 15:03 UTC
How would I go about deciphering the thing after $val? use rxrx, Re^2: ppiwx / wxPPI / wxppixregexp ppixregexplain.pl wxPPIxregexplain.pl PPIx::Regexp::xplain because YAPE::Regex::Explain is dead, Re^4: My Favourite Regex Tools (Was: Parsing a Variable Format String)... so you get ^ # Match only if at start of string (or line) [ ]+ # Match any of the listed characters, one-or- +more times (as many as possible) Pin # Match a literal sequence ("Pin") : # Match a literal ':' character [ ]* # Match any of the listed characters, zero-or +-more times (as many as possible) ( # The start of a capturing block ($1) [^\.]* # Match any character not listed, zero-or-m +ore times (as many as possible) ) # The end of $1 \. # Match a literal '.' character ( # The start of a capturing block ($2) [^ ]* # Match any character not listed, zero-or-m +ore times (as many as possible) ) # The end of $2 [ ]* # Match any of the listed characters, zero-or +-more times (as many as possible) out # Match a literal sequence ("out") [download] or Read more... (14 kB)	[reply] [d/l] [select]