There are certain regulations on carrying a knife in some regions. I wonder what are effective alternatives that can be used in knife's stead, and that can be considered cold weapons.
As no specific region was specified in the question, this answer applies exclusively to Canada. Perhaps similar provisions exists in other legal systems, however.
In Canada, the law allows you to openly carry a knife of pretty much any size, as long as it conforms to these points of the Criminal Code Article 84 :
Prohibited weapon means
(a) a knife that has a blade that opens automatically by gravity or centrifugal force or by hand pressure applied to a button, spring or other device in or attached to the handle of the knife, or
(b) any weapon, other than a firearm, that is prescribed to be a prohibited weapon
As the first item implies, any straight knife or knife that opens from pressure to the blade only is legal, with no size restriction.
The prohibited cold weapons list includes :
- Tear gas, sprays or other similar items
- Kusari or Manrikigusari
- bladed rings (as in weaponized jewellery)
- Crossbows longer than .5m or that can be fired with a single hand
- push daggers
- any item smaller than 30cm that conceals a blade or spike of any kind
- spiked wristbands
- telescoping clubs
- morning stars
- brass knuckles made of metal
Basically, if it's not on that list, it's legal to own one. Notice the emphasis on own... In Canada, use and intent are considered when weapons are involved, as far as the Criminal Code Article 88 is concerned :
Every person commits an offence who carries or possesses a weapon, an imitation of a weapon, a prohibited device or any ammunition or prohibited ammunition for a purpose dangerous to the public peace or for the purpose of committing an offence.
The common interpretation is that if you are carrying weapons for the practice of martial arts or for mundane uses (i.e. that knife on your belt with an 8" blade is for woodworking and general bushcraft, opening sealed boxes, etc.), it's 100% legal. However, if you are carrying for combat purposes, even if it is self-defence, it is deemed "dangerous to the public peace" and you could be arrested. Obviously, carrying a weapon with the intent of committing a crime is prohibited.
Nonetheless, some interesting stuff is still legal to own in Canada. For instance, pretty much any straight knife is OK, as well as unassisted opening folded knives. Metallic and wooden clubs are fine, too, as long as they are not telescopic (it's apparently OK for law enforcement, though). Metallic brass knuckles are prohibited, but not hard plastic or ceramic ones. Bows are apparently perfectly fine, too!
As long as you remember that these items are utilitarian in nature or part of your personal collection, they are perfectly fine. Just never use them as a weapon or state that you could intend to do so if you felt threatened...
As a matter of fact, I used to regularly transport swords of various kinds in the Montreal metro system, going to demonstrations and other martial arts related activities. I was questioned more than once by the local law enforcement, but was never fined or arrested, nor were my weapons confiscated. The only thing that was said to me was that I should wrap them in a piece of cloth, as it becomes more apparent that I'm only carrying the weapons, and not actively prepared to use them. After I started doing so, I was never bothered again, even if I sported the classic anime look with an obvious bundle of swords strapped to my back.
This is going to depend highly on your particular region's laws, be them at the city, county, state, province, or country level. And the laws change from time to time. It's hard to know for sure what's allowed.
The other thing to keep in mind is that even if a weapon is legal, if you go to court on homicide or assault charges, even if it was clearly self-defense, juries are often prejudiced against the use of weapons, particularly if your attacker is unarmed. They can easily rule against you in favor of the prosecution.
Oddly enough, I've heard that guns are the only exception (in the U.S. and other countries where gun ownership is legal and encouraged). They say juries tend to be okay with someone using guns to protect themselves. Not knives, for example. If you use a knife, even legally, the jury may see it as something only a "thug" would be carrying. Only guns are something an "upright" citizen would carry for self-defense.
The best advice I can give anyone who wants to carry a weapon for self-defense that's not a gun is to carry something that isn't a weapon but can be used that way. This has to be something you might have with you at the time and have a good reason to have it other than for self-defense.
For example, a very strong, heavy umbrella. Or a cane or a walking stick. Or even a strong, metal pen with a hard, blunt point to it. Drum sticks. Belts, ropes, and electrical cables can be used defensively. Screw drivers and wrenches. Steel tipped boots. Etc.
If you're a woman carrying a purse, think about keeping rolls of pennies or nickels at the bottom of the purse so you can flail your purse at an attacker. Guys might want to also have rolls of pennies, nickels, or quarters in your jacket pockets to use while punching.
You get the idea.
Legally, that's more justifiable than having an actual weapon on your person and using it. But, again, this depends on your local laws. It often doesn't matter that you used something that wasn't actually a weapon. If you used it "as" a weapon, that may be the only thing that matters, legally.
So you really have to be aware of your own local laws.
The only other thing I'd suggest looking at is pepper spray. In fact, I'd go so far as to say get that before thinking about the improvised non-weapons I mentioned above. It's easily justified in courts as a pure self-defense weapon. It doesn't have the stigma that knives do, being associated with criminals and "thugs". Pepper spray is common, cheap, effective, and legal - in the U.S. anyway, if over 18 years of age.
But make sure you know how to use your pepper spray and go through drills every now and then. You don't want to have to fidget with it to get it to work when you need it. You should know how to aim it and do some target practice with it. It should be kept out of reach of children. There should be a safety lock to prevent it from accidentally discharging. Etc.
All of the states in the U.S. allow pepper spray without a license now (or will as of this July it seems), but there are some regulations you have to abide by on a per-state basis. Other countries may or may not allow pepper spray. Many countries only allow law enforcement officers to have pepper spray. You can check this link for info on your particular location:
But be careful not to use pepper spray without a good reason. Just because it's legal doesn't mean you can't be sued or imprisoned for using it.
That's true of everything else I mentioned as well. If you use an umbrella or a rolled up newspaper against someone, for example, and they press charges against you with the police, you might be imprisoned for battery with a weapon.
Hope that helps.
I'll give you both the "ideal but still probably illegal" answer, and the more realistic answer.
Smashing weapons - still probably illegal
If you can't have edged weapons, a stick or club is a great tool. It gives you reach, leverage, and can smash bones. If you want a short pocket weapon, then you want brass knuckles.
Personally, if I had the choice, I'd take brass knuckles over a knife, because many people report not feeling pain from stabs or cuts, but broken bones destroy the structure people need to keep attacking you. If you don't have the training to incapacitate someone correctly and they bleed out 20 minutes after they've already killed you, it doesn't help you any.
However, these things tend to be just as outlawed as knives so the point is moot.
The practical option
To most of my friends I recommend a flashlight. You can get metal flashlights that have good heft, and range in size from pocket flashlights to foot long "regular" flashlight sizes, which are generally acceptable as normal tools to carry or have in your car. And frankly, you'll use them for actual flashlight duty, so it's an honest claim.
I carry a small 7 inch pocket flashlight that allows me to strike with either a hammerfist or a stabbing motion to strike with either side of the flashlight. Most of the blade techniques I've learned from silat carry over fairly well, provided you train to go for bone smashes and with more force than you would use for a knife.
Don't get the kind with 101 "features" like "tactical edges" or such. Solid construction, heavy weight are what make this work, and all the extras move it from being a tool into a weapon as far as most people are concerned.
Your local laws, interpretation, and bias
Obviously, this also filters through your local regulations. Those also filter through your local law enforcement, which may be much more arbitrary than the actual laws itself. Given that for some people, they can't walk down the street with a soda can without police shooting them, carrying a knife, even if legal, would be out of the question when their primary goal is what makes them safer.
Just carry a pen. It can save your life. If your attacker has a knife, honestly I'd say forget the rules; you are evening your odds or otherwise going 6 ft under. You can legally carry a pen anywhere you want, including into a court room. A simple fountain pen is THE ultimate concealed weapon. I have one in my car and on me at all times. Don't just use it for nothing or flash it around. We know from very early (well, some of us) that it's to your best advantage that other people don't even know you practice martial arts! Good luck, I hope for our sake we don't have to use it. Would it work? Um, yeah, it would.
N.B. don't go buying a "combat pen" or something stupid like that. You're hust fine with a good old-fashioned ball-point pen. Good luck explaining the "combat pen" stupidity to a judge! Hope that helped in some way.