A small detail on the syntax of field list markers

While spelunking in the code, I came across the following regex
(docutils.parsers.rst.states.Body.patterns["field_marker"]) defining
the syntax of field list names and their surrounding colons:

    :(?![: ])([^:\\]|\\.|:(?!([ `]|$)))*(?<! ):( +|$)

I wonder if the final lookbehind assertion might not be slightly
incorrect? The field name is not allowed to end with a space, which is
fine, but given the allowance for backslash escapes one would normally
expect ":foo\ :" to be a permitted (if odd) marker. However, given
that (?<! ) unconditionally looks for a single space before the
terminating colon it is in fact rejected, and backslashes have a
somewhat puzzling behaviour in field names.

I couldn't find any indications in docs or tests either way for what
is intended, but amending the assertion to be (?<![^\\] ) would yield
a more intuitive syntax I think.