I'm quite curious to know whether a footballer (any position) can play the game wearing a cap?I haven't noticed such a situation yet but would love to know whether it is possible or not. PS:Though I think they would not prefer wearing it as it would not allow them to head efficiently but still I'd love to know
In Law 4: The Players’ Equipment; Paragraph 4: Other equipment of Laws of the Game it's stated that:
Non-dangerous protective equipment, for example headgear, facemasks and knee and arm protectors made of soft, lightweight padded material is permitted as are goalkeepers’ caps and sports spectacles.
Where head covers are worn, they must:
- be black or the same main colour as the shirt (provided that the players of the same team wear the same colour)
- be in keeping with the professional appearance of the player’s equipment
- not be attached to the shirt
- not be dangerous to the player wearing it or any other player (e.g. opening/ closing mechanism around neck)
- not have any part(s) extending out from the surface (protruding elements)
Hence, all types of goalkeeper caps are permitted, as they are explicitly mentioned and it's worth to mention that it's a new inclusion as you can see from here. Probably, referees won't allow field players to use caps as they have protruding shapes and can be considered dangerous when fighting for headed balls, and anyways referees have an authority:
to allow or not to allow a player to wear certain clothing or equipment
as it is stated in Law 5: The Referee; Paragraph 6: Liability of Match Officials, so it can also depend on referee or his current mood.