E:\>perl -MYAPE::Regex::Explain -le"print YAPE::Regex::Explain->new('.+@.+\..+')->explain"
The regular expression:

(?-imsx:.+@.+\..+)

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):
----------------------------------------------------------------------
  .+                       any character except \n (1 or more times
                           (matching the most amount possible))
----------------------------------------------------------------------
  @                        '@'
----------------------------------------------------------------------
  .+                       any character except \n (1 or more times
                           (matching the most amount possible))
----------------------------------------------------------------------
  \.                       '.'
----------------------------------------------------------------------
  .+                       any character except \n (1 or more times
                           (matching the most amount possible))
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------