Perl regular expressions - regex

Question

I'm reading some code that involves regular expression and having some trouble.
Can someone please explain it and give an example of text it would parse?
if(/\|\s*STUFF(\d+)\s*\|\s*STUFF(\d+)/)
{
$a = $1;
$b = $2;
}

Answer 1

One string it matches against is |STUFF1|STUFF2.
YAPE::Regex::Explain
(?-imsx:\|\s*STUFF(\d+)\s*\|\s*STUFF(\d+))
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):
----------------------------------------------------------------------
\| '|'
----------------------------------------------------------------------
\s* whitespace (\n, \r, \t, \f, and " ") (0 or
more times (matching the most amount
possible))
----------------------------------------------------------------------
STUFF 'STUFF'
----------------------------------------------------------------------
( group and capture to \1:
----------------------------------------------------------------------
\d+ digits (0-9) (1 or more times (matching
the most amount possible))
----------------------------------------------------------------------
) end of \1
----------------------------------------------------------------------
\s* whitespace (\n, \r, \t, \f, and " ") (0 or
more times (matching the most amount
possible))
----------------------------------------------------------------------
\| '|'
----------------------------------------------------------------------
\s* whitespace (\n, \r, \t, \f, and " ") (0 or
more times (matching the most amount
possible))
----------------------------------------------------------------------
STUFF 'STUFF'
----------------------------------------------------------------------
( group and capture to \2:
----------------------------------------------------------------------
\d+ digits (0-9) (1 or more times (matching
the most amount possible))
----------------------------------------------------------------------
) end of \2
----------------------------------------------------------------------
) end of grouping
----------------------------------------------------------------------

Answer 2

\|\s*STUFF(\d+)\s*\|\s*STUFF(\d+)
\| look for a literal pipe character |.
\s* look for any number (zero or more) whitespace characters.
STUFF look for the string STUFF
(\d+) look for any number of digits (one or more), and save them to $1.
\s* look for any number of whitespace characters (zero or more)
then repeat once, and save the next digit sequence in $2.
If the regex matches, we know that $1 and $2 must be defined (i.e. they have a value).
In that case, we assign $1 to the variable $a and $2 to $b.
As no explicit string to match against is provided, the $_ variable is implicitly used.
Example text:
foo bar |STUFF123|STUFF456 baz bar foo
and
foo |
STUFF0
|STUFF1234567890bar