$string =~ s{

    # match to just before valid lt in a valid bt group.
    (?:
      # matching already started: just after some valid lt:
      # match to next lt.
      # (per perlop, \G must be first in regex.)
      \G (?! \A)  (?&TO_NEXT_LT_IN_THIS_OR_NEXT_VALID_BT_GROUP)
      |
      # at match start: match to first lt in bt group
      \A (?&TO_FIRST_LT_IN_VALID_BT_GROUP)
    )

    # grab and replace lt.
    \K          # ignore everything matched so far
    (?&TRU_LT)  # replace this

    (?(DEFINE)

      # un-unescaped assertion.
      (?<UNESCAPED> (?<! (?<! \\) \\) )
      # an un-unescaped backtick: may start/end a bt group.
      (?<TRU_BT>    (?&UNESCAPED) `)
      # an un-unescaped less-than: may be replaced.
      (?<TRU_LT>    (?&UNESCAPED) <)
      # any character NOT a true backtick.
      (?<NOT_BT>    (?! (?&TRU_BT)) .)
      # any character NOT a true backtick and also NOT an lt.
      (?<NOT_BT_LT> (?! (?&TRU_LT)) (?&NOT_BT))
      # a bt-group NOT containing an lt.
      (?<EMPTY_BT>  (?> (?&TRU_BT) (?&NOT_BT_LT)*+ (?&TRU_BT)))
      # non-bt-group-with-lt stuff to ignore.
      (?<IGNORE>    (?: (?&NOT_BT)*+ (?&EMPTY_BT)*+)*+)

      # to first valid lt in first succeeding valid bt group.
      # assume matching starts OUTside any valid bt group.
      (?<TO_FIRST_LT_IN_VALID_BT_GROUP>
        (?&IGNORE)                    # ignore non-group stuff
        (?&TRU_BT) (?&NOT_BT_LT)*+    # positioned just before lt
        (?= (?&NOT_BT)++ (?&TRU_BT))  # rest is valid bt
        )

      # to next or first lt in this or next valid bt group.
      # assume matching starts INside a valid bt group.
      (?<TO_NEXT_LT_IN_THIS_OR_NEXT_VALID_BT_GROUP>
        # match up to either an lt or a bt (or end of string).
        (?&NOT_BT_LT)*+
        # if a bt, end bt group, match to lt in next valid bt group.
        (?: (?&TRU_BT) (?&TO_FIRST_LT_IN_VALID_BT_GROUP))?+
        )

      )  # end (DEFINE)

    }
    {$replace}xmsg;