C:\>perl -e "use YAPE::Regex::Explain; $REx = '?<[ ]*LOGIN[ ]?'; print $exp = YAPE::Regex::Explain->new($REx)->explain"
The regular expression:



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):
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------

C:\>perl -e "use YAPE::Regex::Explain; $REx = qr/?<[ ]*LOGIN[ ]?/; print $exp = YAPE::Regex::Explain->new($REx)->explain"
Quantifier follows nothing in regex; marked by <-- HERE in m/? <-- HERE <[ ]*LOGIN[ ]?/ at -e line 1.