My friends and I have been playing around with regular expressions, and we ran into something that we haven't been able to explain.

Here is some test code that shows the "phenomena".

#! G:\Perl\bin\perl.exe

$test_string="x"; 

#
# This pattern matches once, and it matches
# after the "x", not before.
# 
@test = ($test_string =~ /^x*/g ); 
$NumMatch=@test;

print "Test String\t>$test_string<\n";
print "Prematch\t>$`<\n";
print "Match\t\t>$&<\n";
print "Postmatch\t>$'<\n";
print "Num Matches\t>$NumMatch<\n";
print "Match Arrary:\n";
foreach $m (@test) {
   print ">$m<\n";
}


print "\n";
print  "-" x 10;
print "\n\n";


# 
# This pattern matches twice, and 
# the second match is after the "x"
# 
@test = ($test_string =~ /x*$/g ); 
$NumMatch=@test;

print "Test String\t>$test_string<\n";
print "Prematch\t>$`<\n";
print "Match\t\t>$&<\n";
print "Postmatch\t>$'<\n";
print "Num Matches\t>$NumMatch<\n";
print "Match Arrary:\n";
foreach $m (@test) {
   print ">$m<\n";
}

__END__
[download]

The above code produces the following output.

Test String     >x<
Prematch        >x<
Match           ><
Postmatch       ><
Num Matches     >1<
Match Arrary:
>x<

----------

Test String     >x<
Prematch        >x<
Match           ><
Postmatch       ><
Num Matches     >2<
Match Arrary:
>x<
><
[download]

Here is why we are confused:

Why does the first pattern match after the "x". I understand that "x*" is able to match nothing, but shouldn't it match the nothing before the "x", rather than the nothing after the "x".

The reason I think it should match before the "x" is that I use the "^" assertion. Also, Camel2, p61 says:

"... any regular expression that can match the null string is guaranteed to match at the leftmost position in the string."

Also, why does the second pattern "x*$" match twice when the first pattern matched only once. It seems as though they should either both match once or both match twice.

Thanks,

Carl-Joseph

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