Laws - they're made to be broken. Especially the ridiculous ones.
just don't get on the wrong line at the supermarket or you could be subject to some extreme browbeating =)
Here in PA, prohibited offensive weapons are defined as any "dagger, knife, razor or cutting instrument, the blade of which is exposed in an automatic way by switch, push-button, spring mechanism, or otherwise". There is no legal or illegal length, per se. On the other hand, the city of Philadelphia bans all knives not being used specifically for work:
§10-820. Cutting Weapons in Public Places. [180]
(1) Definition.
Cutting Weapon. Any knife or other cutting instrument which can be used as a weapon that has a cutting edge similar to that of a knife. No tool or instrument commonly or ordinarily used in a trade, profession or calling shall be considered a cutting weapon while actually being used in the active exercise of that trade, profession or calling.
(2) Prohibited Conduct. No person shall use or possess any cutting weapon upon the public streets or upon any public property at any time.
(3) Penalty. The penalty for violation of this section shall be a fine of not less than three hundred (300) dollars and imprisonment of not less than ninety days.
I leave my knife at home when I am in the city, our firearm carry laws are a lot less prohibitive.