beside the wisdom received from brother choroba you can find interesting the read of Using Look-ahead and Look-behind.

Also the precious resource of wisdom Modern Perl (the free book!) has a chapter dedicated to regexes: look for the Assertions paragraph in chapter 6

L*

Thanks Discipulus for the reference. Will go through that

As further references, you may find perlre, perlretut, and perlrequick helpful in general. Also, in the Tutorials of this site, the section Pattern Matching, Regular Expressions, and Parsing.

---

Give a man a fish:  <%-{-{-{-<

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

while (<DATA>) {
    print "$. ok\n" if /^foo      # "foo"
                     _            # followed by "underscore"
                     (?:bar|test) # followed by "bar|test" (bar or tes
+t)
                     _            # followed by "underscore"

                     ((?:\w+\.)+) # followed by alhpnum string with
                                  # dot at end (like foo10.1.1.1)

                     \1?          # followed by one or zero occurences
                                  # of same alhpnum string (like
                                  # foo10.1.1.1) followed by dot

                     \w{1,10}     # followed by 1-10 characters,
                                  # containing at least one digit and
                                  # one letter and underscore (e.g.
                                  # TEST_test, TEST_test2,
                                  # TEST1_test2)

                     \.txt$       # ends with .txt
                    /x;
}

__DATA__
foo_bar_foo10.1.1.1.TEST.txt
foo_test_foo10.1.1.1.foo10.1.1.1.TEST_test.txt
[download]

To test the condition "at least one digit and one letter and underscore", I'd probably capture the group and test these conditions independently:

/\d/ && /[[:alpha:]]/ && /_/
[download]

($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,
[download]

Thank you very much choroba. This works !!!

One last thing I need to extend to below match also

foo_bar_foo10.1.1.1.TEST.txt
foo_test_foo10.1.1.1.foo10.1.1.1.TEST_test.txt
foo_test_foo10.1.1.1.foo10.1.1.1.txt 
foo_test_foo10.1.1.1.foo10.1.1.1.TEST-1.txt
[download]

Third string is not a correct match but this is displayed as "Ok" whereas fourth doesn't seems ok but I want to match this also

for "foo_test_foo10.1.1.1.foo10.1.1.1.TEST-1.txt" I modified the expression as (\w\-{1,10}) and that works ... Is this correct way ?

trying to achieve the third format within the same format - as Nok

Output for now 1 ok 2 ok 3 ok 4 ok

Does this approach allows to use variable inside the regex format ?

Can I declare like my $var = foo10.1.1.1 and use this in regex format ?

Yes it does ... found answer for $var declaration I replaced  ((?:\w+\.)+) as ((?:($var)+\.)+) ==> Is this correct too

Last thing I am not able to get

3 is not correct .... I want to filter this out

OR

foo_test_foo10.1.1.1.foo10.1.1.1.txt - How to match only this in a separate regex

I don't understand your requirements. It now seems to me none of the examples is correct, as TEST and TEST_test don't contain at least one digit, and TEST-1 doesn't contain at least one underscore. Changing the dash to underscore in the last makes it pass:

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

while (<DATA>) {
    if (my ($alnum, $constrained)
        = /^foo          # "foo"
           _             # followed by "underscore"
           (?:bar|test)  # followed by "bar|test" (bar or test)
           _             # followed by "underscore"

           ((?:\w+\.)+?) # followed by alhpnum string with
                         # dot at end (like foo10.1.1.1)

           \1?           # followed by one or zero occurences
                         # of same alhpnum string (like
                         # foo10.1.1.1) followed by dot

           (\w{1,10})    # followed by 1-10 characters,
                         # containing at least one digit and
                         # one letter and underscore (e.g.
                         # TEST_test, TEST_test2,
                         # TEST1_test2)

           \.txt$        # ends with .txt
          /x) {

        print "<$alnum | $constrained> $. ok\n"
            if 3 == grep $constrained =~ $_,
                qr/_/, qr/\d/, qr/[[:alpha:]]/;
    }
}
__DATA__
foo_bar_foo10.1.1.1.TEST.txt
foo_test_foo10.1.1.1.foo10.1.1.1.TEST_test.txt
foo_test_foo10.1.1.1.foo10.1.1.1.txt
foo_test_foo10.1.1.1.foo10.1.1.1.TEST-1.txt
foo_test_foo10.1.1.1.foo10.1.1.1.TEST_1.txt
[download]

Note that I changed the quantifier in the first capturing group to frugal to avoid matching both occurrences of $alnum as one without repetition. Nevertheless, in the third case, the whole substring foo10.1.1.1.foo10.1.1. corresponds to $alnum, it's not repeated, and $constrained is just 1 .

($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,
[download]

One last thing I need to extend to below match also

When asking for help on regexes, please provide all the possible test cases. Also, you can use a module like Test::More - first, write as many test cases as possible, then work on your regex until all the tests pass, or post the code here if you have trouble.

use warnings;
use strict;
use Test::More;

my $regex = qr/foo/;

  like "foobar", $regex;
unlike "quzbaz", $regex;
# more test cases here

done_testing;
[download]