Give us some code we can run. See I know what I mean. Why don't you?

Update: What do you expect /^[5]+$/ to match? What does each component of the expression do? To save us all a little time these are the interesting components:

• ^
• [5]+
• $

Further to GrandFather's remarks:   The regex /^[5]+$/ does not match the string '503009496', but is there any other string it would match? What about the strings

5
55
555
555555555
[download]

If you find some strings that match, how would they and the strings you know do not match interact with the regex components mentioned by GrandFather? Please see perlre, perlretut, perlrequick and perlreref.

you already received good hints. A minimal read of the manual is something to do.

Here the explanation of your regex:

perl -MYAPE::Regex::Explain -e "print YAPE::Regex::Explain->new(qr/^[5
+]+$/)->explain()"

The regular expression:

(?-imsx:^[5]+$)

matches as follows:

NODE                     EXPLANATION
----------------------------------------------------------------------
(?-imsx:                 group, but do not capture (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  ^                        the beginning of the string
----------------------------------------------------------------------
  [5]+                     any character of: '5' (1 or more times
                           (matching the most amount possible))
----------------------------------------------------------------------
  $                        before an optional \n, and the end of the
                           string
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------
[download]

L*

Thanks everyone for your responses, i have resolved the issue with below regex.

/^[5](?=\d)

Let's analyze this regex:

• ^ indicates the pattern anchors to the beginning of the string.
• [...] is a character class. It allows you to create an alternate of choice where a single character in the target string matches any one of the characters within the class.
• [5] is a character class comprised of only one character, which isn't very useful in this case. You're using a "match any of these" construct but asserting that "these" are any of the following: "5". You could do away with the character class and just put a 5 there.
• + is a quantifier. It says match one or more of the previous atom. In this case, match [5] one or more times.
• $ is an anchor to the end of string.

Taken in full, you're saying starting from the beginning and ending at the end of the target string, match one or more 5 characters, and nothing else. You didn't allow for the trailing digits. Since you anchor to the end of the string, you're asserting that the string can only contain a bunch of 5s.

If you want to allow for eight trailing digits that are not 5s, you need to specify that too:

Now you're saying the target has to start with a 5 and then must match eight more digits, at which point the target string must end.

#!/usr/bin/perl

use strict; # https://perlmonks.org/?node_id=11132725
use warnings;

while( <DATA> )
  {
  print /(?<!\d)5\d{8}(?!\d)/ ? '   ' : 'not', " found in : ", $_;
  }

__DATA__
for me. "503009496" and i w
for me. "603055496" and i w (6)
for me. "703509496" and i w (7)
for me. "53009496" and i w (too short)
for me. "5093009496" and i w (too long)
for me. "503009496" and i w
[download]

Outputs:

    found in : for me. "503009496" and i w
not found in : for me. "603055496" and i w (6)
not found in : for me. "703509496" and i w (7)
not found in : for me. "53009496" and i w (too short)
not found in : for me. "5093009496" and i w (too long)
    found in : for me. "503009496" and i w
[download]

my $var = '503009496';

my $i = index($var, '5');
[download]