To decipher foreign regexes, I also use YAPE::Regex::Explain (which is also free because it is on CPAN :). Using the OP's original regex (excluding the modifiers, /ig): 
use warnings;
use strict;
use YAPE::Regex::Explain;

my $re = '<container><a>(.+?)<\/a><b>(.+?)<\/b><\/container>(<\?query\
+?>)??';
print YAPE::Regex::Explain->new($re)->explain;

[download]

produces this output: 

The regular expression:

(?-imsx:<container><a>(.+?)<\/a><b>(.+?)<\/b><\/container>(<\?query\?>
+)??)

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):
----------------------------------------------------------------------
  <container><a>           '<container><a>'
----------------------------------------------------------------------
  (                        group and capture to \1:
----------------------------------------------------------------------
    .+?                      any character except \n (1 or more times
                             (matching the least amount possible))
----------------------------------------------------------------------
  )                        end of \1
----------------------------------------------------------------------
  <                        '<'
----------------------------------------------------------------------
  \/                       '/'
----------------------------------------------------------------------
  a><b>                    'a><b>'
----------------------------------------------------------------------
  (                        group and capture to \2:
----------------------------------------------------------------------
    .+?                      any character except \n (1 or more times
                             (matching the least amount possible))
----------------------------------------------------------------------
  )                        end of \2
----------------------------------------------------------------------
  <                        '<'
----------------------------------------------------------------------
  \/                       '/'
----------------------------------------------------------------------
  b><                      'b><'
----------------------------------------------------------------------
  \/                       '/'
----------------------------------------------------------------------
  container>               'container>'
----------------------------------------------------------------------
  (                        group and capture to \3 (optional
                           (matching the least amount possible)):
----------------------------------------------------------------------
    <                        '<'
----------------------------------------------------------------------
    \?                       '?'
----------------------------------------------------------------------
    query                    'query'
----------------------------------------------------------------------
    \?                       '?'
----------------------------------------------------------------------
    >                        '>'
----------------------------------------------------------------------
  )??                      end of \3 (NOTE: because you're using a
                           quantifier on this capture, only the LAST
                           repetition of the captured pattern will be
                           stored in \3)
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------

[download]

While helpful, I still did not grok the ?? usage :(

In reply to Re^2: Special Variables in Regular Expression by toolic
in thread Special Variables in Regular Expression by fiat

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