use YAPE::Regex::Explain;
print YAPE::Regex::Explain ->new(
    qr/\[b\]([^\[\/b\]]+)/
)->explain;
__END__
The regular expression:

(?-imsx:\[b\]([^\[/b\]]+))

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):
----------------------------------------------------------------------
  \[                       '['
----------------------------------------------------------------------
  b                        'b'
----------------------------------------------------------------------
  \]                       ']'
----------------------------------------------------------------------
  (                        group and capture to \1:
----------------------------------------------------------------------
    [^\[/b\]]+               any character except: '\[', '/', 'b',
                             '\]' (1 or more times (matching the most
                             amount possible))
----------------------------------------------------------------------
  )                        end of \1
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------