Loosely used to mean `in accordance with all the relevant
rules', esp. in connection with some set of constraints defined
by software. "The older =+ alternate for += is no longer legal
syntax in ANSI C." "This parser processes each line of legal
input the moment it sees the trailing linefeed." Hackers often
model their work as a sort of game played with the environment
in which the objective is to maneuver through the thicket of
`natural laws' to achieve a desired objective. Their use of
`legal' is flavored as much by this game-playing sense as by the
more conventional one having to do with courts and lawyers.
Compare language lawyer, legalese.