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W d, 5 bytes

(Possibly the only <=5-byter that does not use a prime or number factorization built-in?) (The two spaces here represent a tab)

‡T B⑸ 

repr'd:'‡T\tB⑸'

Unpacked:

m!Wk2= 

Explanation

This is really difficult to read, so an explanation will be nice.

 W % (implicit) Foreach the inclusive range from 1 to the input: bam % The "bam" magic! Just kidding. Evaluates b % a % Since b is out-of-bounds, this takes the upper-level input % as the operand. ! % Remove those that aren't valid multiples. k % Is the length of the array ... 2= % ... equal to 2? % i.e. primes have exactly 2 factors: 1 and itself 

W d, 5 bytes

(Possibly the only <=5-byter that does not use a prime or number factorization built-in?) (The two spaces here represent a tab)

àTéBΦ 

repr'd:'‡T\tB⑸'

Unpacked:

m!Wk2= 

Explanation

This is really difficult to read, so an explanation will be nice.

 W % (implicit) Foreach the inclusive range from 1 to the input: bam % The "bam" magic! Just kidding. Evaluates b % a % Since b is out-of-bounds, this takes the upper-level input % as the operand. ! % Remove those that aren't valid multiples. k % Is the length of the array ... 2= % ... equal to 2? % i.e. primes have exactly 2 factors: 1 and itself 

W d, 5 bytes

(Possibly the only <=5-byter that does not use a prime or number factorization built-in?)

⊂ℝ]Aא 

Unpacked:

m!Wk2= 

And, just for the lols (explanation for this program is also below, without the implicitly provided ba):

W d, 13 7 bytes

W doesn't have a boring primalty test operation, which is why this program is so long. (IMO a 7-byte program is extremely long.)

“]⑴₳ė⓷⑭ 

Decompressed form:

bam!Wk2= 

Explanation

This is really difficult to read, so an explanation will be nice.

 W % (implicit) Foreach the inclusive range from 1 to the input: bam % The "bam" magic! Just kidding. Evaluates b % a % Since b is out-of-bounds, this takes the upper-level input % as the operand. ! % Remove those that aren't valid multiples. k % Is the length of the array ... 2= % ... equal to 2? % i.e. primes have exactly 2 factors: 1 and itself 

W d, 5 bytes

(Possibly the only <=5-byter that does not use a prime or number factorization built-in?) (The two spaces here represent a tab)

‡T B⑸ 

repr'd:'‡T\tB⑸'

Unpacked:

m!Wk2= 

Explanation

This is really difficult to read, so an explanation will be nice.

 W % (implicit) Foreach the inclusive range from 1 to the input: bam % The "bam" magic! Just kidding. Evaluates b % a % Since b is out-of-bounds, this takes the upper-level input % as the operand. ! % Remove those that aren't valid multiples. k % Is the length of the array ... 2= % ... equal to 2? % i.e. primes have exactly 2 factors: 1 and itself 

W d, 5 bytes

⊂ℝ]Aא 

Unpacked:

m!Wk2= 

And, just for the lols (explanation for this program is also below, without the implicitly provided ba):

W d, 13 7 bytes

W doesn't have a boring primalty test operation, which is why this program is so long. (IMO a 7-byte program is extremely long.)

(Possibly the only <=7-byter that does not use a prime or number factorization built-in?)

“]⑴₳ė⓷⑭ 

Decompressed form:

bam!Wk2= 

Explanation

This is really difficult to read, so an explanation will be nice.

 W % (implicit) Foreach the inclusive range from 1 to the input: bam % The "bam" magic! Just kidding. Evaluates b % a % Since b is out-of-bounds, this takes the upper-level input % as the operand. ! % Remove those that aren't valid multiples. k % Is the length of the array ... 2= % ... equal to 2? % i.e. primes have exactly 2 factors: 1 and itself 

W d, 5 bytes

(Possibly the only <=5-byter that does not use a prime or number factorization built-in?)

⊂ℝ]Aא 

Unpacked:

m!Wk2= 

And, just for the lols (explanation for this program is also below, without the implicitly provided ba):

W d, 13 7 bytes

W doesn't have a boring primalty test operation, which is why this program is so long. (IMO a 7-byte program is extremely long.)

“]⑴₳ė⓷⑭ 

Decompressed form:

bam!Wk2= 

Explanation

This is really difficult to read, so an explanation will be nice.

 W % (implicit) Foreach the inclusive range from 1 to the input: bam % The "bam" magic! Just kidding. Evaluates b % a % Since b is out-of-bounds, this takes the upper-level input % as the operand. ! % Remove those that aren't valid multiples. k % Is the length of the array ... 2= % ... equal to 2? % i.e. primes have exactly 2 factors: 1 and itself