diff --git a/src/core.asm b/src/core.asm
index 9911208..5339070 100644
--- a/src/core.asm
+++ b/src/core.asm
@@ -2,6 +2,8 @@
 
 section .text
 	global	exit
+	global	islower
+	global	isupper
 
 ;----- exit(exit_code) -----;
 ; Exits the program with given exit code
@@ -13,3 +15,35 @@ exit:
 	sub	rsp, SIZE_QWORD
 	mov	rax, NR_exit
 	syscall
+
+;----- islower(c) -----;
+; Checks if given ASCII character is lowercase (a-z)
+; Return value: 1 if ASCII character is lowercase (is in range a-z), 0 otherwise
+; Used registers:
+;	rax*	(ret)
+;	rdi	(arg) Character to check
+islower:
+	xor	rax, rax
+	cmp	dil, 'a'
+	jb	.quit
+	cmp	dil, 'z'
+	ja	.quit
+	mov	rax, 1
+	.quit:
+	ret
+
+;----- isupper(c) -----;
+; Checks if given ASCII character is uppercase (A-Z)
+; Return value: 1 if ASCII character is uppercase (is in range A-Z), 0 otherwise
+; Used registers:
+;	rax*	(ret)
+;	rdi	(arg) Character to check
+isupper:
+	xor	rax, rax
+	cmp	dil, 'A'
+	jb	.quit
+	cmp	dil, 'Z'
+	ja	.quit
+	mov	rax, 1
+	.quit:
+	ret