The legality of using a knife or any other weapon for self-defense varies depending on the jurisdiction and the specific circumstances of the situation. Laws regarding self-defense can differ greatly between countries, states, and even within local jurisdictions.

In general, the use of any weapon, including a knife, for self-defense is typically subject to several legal considerations, which may include:

1. Proportionality: The use of force in self-defense is generally required to be proportionate to the threat faced. This means that the level of force used should not exceed what is reasonably necessary to protect oneself from harm.
2. Imminent Threat: Self-defense is generally justifiable when there is an immediate and imminent threat to one's safety or the safety of others. The threat must be objectively reasonable, meaning that a reasonable person in the same situation would perceive the threat and respond in a similar manner.
3. Reasonable Belief: The person claiming self-defense must have a reasonable belief that the use of force, including the use of a knife, is necessary to prevent imminent harm.
4. Duty to Retreat: Some jurisdictions have laws that require individuals to first attempt to retreat or avoid the threat if it is safe to do so before resorting to using force in self-defense. Other jurisdictions may have "stand your ground" laws that eliminate the duty to retreat in certain situations.

It's important to note that I am an AI language model and not a legal authority. Laws can vary, so it is crucial to consult with local laws, legal professionals, or law enforcement authorities in your specific jurisdiction to understand the applicable self-defense laws and regulations. They can provide guidance and clarify the specific legal provisions regarding the use of weapons, including knives, for self-defense purposes.