In keeping with TMTOWTDI, the following will correctly match an IPv4 address at the start of a scalar: 

/^((\d{1,2}|1\d{2}|2[0-4]\d|25[0-5])\.){3}\2(\s|$)
+/

[download]

And broken down (to make it look even more complicated): 

/         (begin pattern match)
^         (match start of scalar)
(         (begin grouping match of '0.' to '255.')
(         (begin saving match of '0' to '255', to \2)
\d{1,2}|  (match '0' to '99' or...)
1\d{2}|   (match '100' to '199' or...)
2[0-4]\d| (match '200' to '249' or...)
25[0-5]   (match '250' to '255')
)         (end saving match of '0' to '255', to \2)
\.        (match a '.')
)         (end grouping match of '0.' to '255.')
{3}       (match '0.' to '255.' 3 times)
\2        (match '0' to '255')
(\s|$)    (match whitespace or scalar end (when \n absent))
/         (end pattern match)

[download]

======= BTW, that:

perl -le "s,,reverse killer,e,y,rifle,lycra,,print"

...is pretty /swe{3,}t/. :)

LaurenceHunter => "Life is loud, life is 60p/min."

In reply to Re: Clarify this Regular Expression by Anonymous Monk
in thread Clarify this Regular Expression by dru145

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