problem using regexp

[artifact]

how do i match a particular alphabet using regexp? I cannot print the second occurrence of 'a' in my string in the following piece of code,as a beginner I cannot find where the problem is and why it is happening?

#! usr/bin/perl
use warnings;
use strict;
my @vars;
$_='1: A a A silly a sentence a (495,a) *BUT* one a which is a will be
+ useful. (3)';
if (@vars= /(\s)(a)(\s)/ig){
    print "the text matches the pattern '\$pattern'.\n";
    foreach (@vars) {
        print "Group: $_\n";
    }
}
else {
print " it was not found. \n";
}
print "@vars\n";

[AnomalousMonk]

if (@vars= /(\s)(a)(\s)/ig){
    ...
}

Another approach to handling overlapping matches (and, IMHO, better because less error-prone and better integrated into the general regex expression than the manipulation of pos) is the use of a look-ahead assertion to wrap capturing groups: see  "(?=pattern)" in the Look-Around Assertions sub-section of perlre Extended Patterns. Replacing
    /(\s)(a)(\s)/ig
with
    /(?=(\s)(a)(\s))/ig
in the code quoted above from the OP produces the desired capture (tested).

[artifact]

thanks for replying so fast ...and also it encouraged me to find different techniques.

[choroba]

The first match eats up the space. The second 'a' therefore is not matched, because there is no space before it. If you do not want the space to be eaten, you can use a look-around:

/\s(a)(?=\s)/ig

[artifact]

thanks ,it works!!

[Grimy]

The problem here is overlapping matches. The space between the first and the second occurence of /a/i is part of both possible matches. Since it is gobbled by the first match, the second /a/i doesn't match.

It is possible to "rewind" your regex by assigning to pos. See 'pos' in perldoc.

#! usr/bin/perl
use strict;
use warnings;

$_=' A a A ';
while (/(\s)(a)(\s)/ig) {
    print "The text matches the pattern '$&' at position " . pos() . "
+.\n";
    # uncomment this to rewind
    #pos() = pos() - 1;
}

[artifact]

it works!! it also made me learn few other concepts.