I'm also very intersted in anything you have to say about how (if at all) you can document or comment a Regular Expression, so it at least looks like commented modem noise instead of just modem noise.
See perlre /x modifier.

Also, courtesy of YAPE::Regex::Explain, here is your /^((\^[%A-Za-z\d]+)($|=|\(bunch_of_ugly_subscripts\)))/ with comments: 

(?x-ims:               # group, but do not capture (disregarding
                       # whitespace and comments) (case-sensitive)
                       # (with ^ and $ matching normally) (with . not
                       # matching \n):

  ^                      # the beginning of the string

  (                      # group and capture to \1:

    (                      # group and capture to \2:

      \^                     # '^'

      [%A-Za-z\d]+           # any character of: '%', 'A' to 'Z', 'a'
                             # to 'z', digits (0-9) (1 or more times
                             # (matching the most amount possible))

    )                      # end of \2

    (                      # group and capture to \3:

      $                      # before an optional \n, and the end of
                             # the string

     |                      # OR

      =                      # '='

     |                      # OR

      \(                     # '('

      bunch_of_ugly_su       # 'bunch_of_ugly_subscripts'
      bscripts               #

      \)                     # ')'

    )                      # end of \3

  )                      # end of \1

)                      # end of grouping

[download]
UPDATE: Does this CPAN search help?

In reply to Re: MUMPS Array Subscripts Parsing Via RegEx by toolic
in thread MUMPS Array Subscripts Parsing Via RegEx by Clovis_Sangrail

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