[Feature Request] Boost.regex - allow OR operator in look-behind

[Palloxin]

Description

As the title says.

The lib doesnt accept OR operator in lookbehind.
Example:

/\w+(?<=Sublime|Bob)/

doesnt work.

BUT it accepts this:

/\w+(?<=Sublime)(?<=Bob)/

I would like the lib to support the OR operation for the regex look-behind

Other Details

These don't work too:

/\w+(?<=(?:Sublime|Bob))/

/\w+(?<=\s?Sublime)/

[JohnAlt2]

ORs are accepted in lookbehinds so long as each branch is of the same length e.g.

/\w+(?<=Dave|John|.Bob)/

look-behinds needing to be fixed length is a known feature as documented e.g.: https://www.boost.org/doc/libs/1_86_0/libs/regex/doc/html/boost_regex/syntax/perl_syntax.html

_Lookbehind
(?<=pattern) consumes zero characters, only if pattern could be matched against the characters preceding the current position (pattern must be of fixed length).

(?<!pattern) consumes zero characters, only if pattern could not be matched against the characters preceding the current position (pattern must be of fixed length)._

The desired functionality can often instead be created by using an OR of two separate look-behinds:

/\w+(?:(?<=Sublime)|(?<=Bob))/

Or adapt your branches to make them the same length such as:

/\w+(?<=Sublime|....Bob)/