added string.isdigit, islower, isupper, isletter

compiler/res/prog8lib/string.p8

@@ -383,4 +383,55 @@ fail    clc             ; yes, no match found, return with c=0
             rts
         }}
     }
+
+    asmsub isdigit(ubyte character @A) -> bool @Pc {
+        %asm {{
+            cmp  #'0'
+            bcs  +
+            rts
++           cmp  #'9'+1
+            bcc  +
+            clc
+            rts
++           sec
+            rts
+        }}
+    }
+
+    asmsub isupper(ubyte character @A) -> bool @Pc {
+        %asm {{
+            cmp  #'A'
+            bcs  +
+            rts
++           cmp  #'Z'+1
+            bcc  +
+            clc
+            rts
++           sec
+            rts
+        }}
+    }
+
+    asmsub islower(ubyte character @A) -> bool @Pc {
+        %asm {{
+            cmp  #'a'
+            bcs  +
+            rts
++           cmp  #'z'+1
+            bcc  +
+            clc
+            rts
++           sec
+            rts
+        }}
+    }
+
+    asmsub isletter(ubyte character @A) -> bool @Pc {
+        %asm {{
+            jsr  islower
+            bcs  +
+            jmp  isupper
++           rts
+        }}
+    }
 }

compiler/res/prog8lib/virtual/string.p8

@@ -174,4 +174,20 @@ string {
                 return hashcode
         }
     }
-}
+
+    sub isdigit(ubyte character) -> bool {
+        return character>='0' and character<='9'
+    }
+
+    sub isupper(ubyte character) -> bool {
+        return character>='A' and character<='Z'
+    }
+
+    sub islower(ubyte character) -> bool {
+        return character>='a' and character<='z'
+    }
+
+    sub isletter(ubyte character) -> bool {
+        return islower(character) or isupper(character)
+    }
+ }

docs/source/libraries.rst

@@ -266,6 +266,9 @@ Provides string manipulation routines.
 ``upperchar (char)``
     Returns uppercased character.
 
+``isdigit (char)``, ``islower (char)``, ``isupper (char)``, ``isletter (char)``
+    Returns boolean if the character is a numerical digit 0-0, lowercase letter, uppercase letter, or any letter.
+
 ``startswith (string, prefix) -> bool``
     Returns true if string starts with prefix, otherwise false