From the laws of the game: "Any player who lunges at an opponent in challenging for the ball from the front, from the side or from behind using one or both legs, with excessive force or endangers the safety of an opponent is guilty of serious foul play."
Shearer also believes that England players should be allowed to stamp on people and elbow them without reprisals, as long as they are called Alan and play for Saudi Sportswashing MachineShearers question is a valid one. It's foul play for fight with an opponent. What is the foul when fighting with a team-mate? There could be a later suspension for bring the game into disrepute, but I'm not sure what the red card offence is.
For instance, this law posted in another thread ...
... suggests that lunging at a team-mate and endangering the safety of a team-mate is within the bounds of the law. The restriction is explicitly limited to opponents.