ereg("fla(r[gpm]) wobba\1", $foo)

Note the ``\1'' -- is that an octal 1 in my charset, or a backreference?

The backslash-x-number notation is great for hexidecimal, but the backslash-number notation conflicts with the notation for backreferences (in ereg(), e.g.)

Are there backreferences?  The manual refers us to the regex(7) manpage, which does describe them.

Even if there's not, I recommend backslash-zero-octal, as historically, octals are represented with preceding zeros.  Backreferences are positive integers (which mortals usually describe with no leading zero), so there's no conflict.