Since I got bitten with a couple of my public attempts at regex construction, I decided to re-read the pod, japhy's book and take a (private) crack at any regex Q's that came up here in an attempt to improve my skills.

Whats below was started by this [untitled node, ID 192753] SoPW, and I am posting for two reasons.

1. I would like feedback on my mechanism for deriving my regex. The idea being to do a little as necessary (laziness) and to use as few wildcard components as possible for best performance.
2. There are half a dozen questions in the comments (look for the ?????) that I would request explanations or pointers on.

#! perl -w
use strict;
my @site;
push  @site, "<!-- USER $_ - donkey_pusher_$_ -->" for (1..10);

=pod
###  Start with a typical sample

 /<!-- USER 20 - donkey_pusher_6 -->/
 
### Escape anything that might cause a problem (Nothing to do here!) 

 /<!-- USER 20 - donkey_pusher_6 -->/        
 
### Add some anchors if I KNOW that they are true 

 /^<!-- USER 20 - donkey_pusher_6 -->$/ 
 
### Bracket the bit(s) I want to keep. 

 /^<!-- USER 20 - (donkey_pusher_6) -->$/
 
### Substitute appropriate wildcards (Right term?) for the bits I know
+ will change 
### The start/end of html comments have to be fixed pretty much.
### The whitespace could vary, but the /x modifier ??should?? handle t
+hat nicely

 /^<!-- USER \d+ - (\w+) -->$/        
 
### Add any modifiers that might help. 
### /x so each space will match any number or combination of whitespac
+e.
### /o to compile for speed,  ??not clear to me if this is necessary o
+r advantagous
###    if there are no variables to be interpolated in the regex??
### /i incase "USER" might vary in case. 
###    If its not needed, don't! I think it is probably expensive.
 /^<!-- USER \d+ - (\w+) -->$/xio
=cut
# qr// can have some speed benefits, ??when??
my $regex1 = qr/^<!-- USER \d+ - (\w+) -->$/iox;

# I had to remove the /x  else it didn't match SOMETIMES????????
my $regex2 = qr/^<!-- USER \d+ - (\w+) -->$/io;    

for (@site ) {
    
## Standalone with /x on qr//  NEVER matches ?????????    
#    next unless $_ =~ $regex1;    

# This FAILS to match also ???????????
#    next unless m/$regex1/;    

# Adding the /x to $regex2 this way works ok
#    next unless m/$regex2/x;

    next unless $_ =~ $regex2;    # And this works.
    my $userid = $1;
    print $userid,$/;
}

# use the map trick
# using $regex1 with the /x modifier  WORKS ok here!! 

my @users = map { $regex1 ? $1 : () } @site;

my $doc=join "\n", @site; # Simulating slurp mode here!!!
study $doc; ## Could give big boost on long strings?

# Again, adding the /x modifier here....
my $regex3 = qr/^<!-- USER \d+ - (\w+) -->$/oi; 

#my @users2 = $doc =~ /$regex3/mgc; # means this FAILS! ??
my @users2 = $doc =~ /$regex3/xmgc; # Adding here it works.

print 'results:', ~~@users, ' ', ~~@users2, $/;
[download]

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What's this about a "crooked mitre"? I'm good at woodwork!

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