Using index when the substring as is a regular expression

[rbi]

Hi,
I would like to locate the position where one out of several possible substrings occours into a string.
say I have strigs like:

some_letters-01-some_other_letters
some_other_letters-03-some_other_letters
...

and I want to set my mask to catch:

 /-(..)-/

It would be like using index STR, SUBSTR, where SUBSTR is not a string, but some regular expression building a mask.
How should this be done?
Thanks in advance,
Roberto

[stephen]

From the perlvar manpage:

@-

$-[0] is the offset of the start of the last successful match. $-[n] is the offset of the start of the substring matched by n-th subpattern, or undef if the subpattern did not match.

Thus after a match against $_, $& coincides with substr $_, $-[0], $+[0] - $-[0]. Similarly, $n coincides with substr $_, $-[n], $+[n] - $-[n] if $-[n] is defined, and $+ coincides with substr $_, $-[$#-], $+[$#-]. One can use $#- to find the last matched subgroup in the last successful match. Contrast with $#+, the number of subgroups in the regular expression. Compare with @+.

This array holds the offsets of the beginnings of the last successful submatches in the currently active dynamic scope. $-[0] is the offset into the string of the beginning of the entire match. The nth element of this array holds the offset of the nth submatch, so $+[1] is the offset where $1 begins, $+[2] the offset where $2 begins, and so on. You can use $#- to determine how many subgroups were in the last successful match. Compare with the @+ variable.

After a match against some variable $var:

$` is the same as substr($var, 0, $-[0]) 
$& is the same as substr($var, $-[0], $+[0] - $-[0]) 
$' is the same as substr($var, $+[0]) 
$1 is the same as substr($var, $-[1], $+[1] - $-[1]) 
$2 is the same as substr($var, $-[2], $+[2] - $-[2]) 
$3 is the same as substr $var, $-[3], $+[3] - $-[3])

Therefore, you can do something like this:

my $q = <<End_Text;
some_letters-01-some_other_letters
some_other_letters-03-some_other_letters
End_Text

while ( $q =~ m{\w+ \-(\d\d)\- \w* }gx ) {
    my $start_index = $-[1];
    print "Mask start index is ", $start_index, "\n";
}

I don't know if this method incurs the same overhead as $`, etc... anybody?

Update: The previous code made loud banging sounds when tested. This code fixes it. Works on 5.6; uncertain if it will work for previous Perls.

stephen

[Corion]

Take a look at the pos() function, it returns the position of the end of the match. But really looks to me like you don't know about the magic variables $1 to $9, which capture your parentheses...

First some codes that does what you ask for :

$s = "Test-01-xxx";
if ($s =~ /-(\d\d)-/g) {
  print "Pos: $pos - ", pos( $s )-3, "\n";
}

And here's some code that does what you need :

$s = "Test-01-xxx";
if ($s =~ /(.*?)-(\d\d)-(.*)/g) {
  print "The string was split into :\n"
  print "Left part : $1\n";
  print "Number part : $2\n";
  print "Right part : $3\n";
}

Update : On rereading your post, I see that you want the start of a RE, so there is no way you can get around solution number two. Here's the necessary merge of method one and two. One method is to use the $& variable, which will slow your program down because Perl will keep track of every match in the $& variable then (but maybe perl does anyway, see here for more info about $&). The other method would be to introduce even more parentheses :

$s = "Test-01-xxx";
# Method 1, possibly slow
if ($s =~ /-(\d\d)-/g) {
  print "Pos: $pos - ", pos( $s )- length( $& ), "\n";
}

# Method 2, possibly slow
if ($s =~ /(-(\d\d)-)/g) {
  print "Pos: $pos - ", pos( $s )- length( $1 ), "\n";
}

Update : See below for stephens info about @- - there's something to learn about Perl every day :-)