Solve Exercise 3-3: expand()

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+#include <stdio.h>
+#include <ctype.h>
+
+/* The C Programming Language: 2nd Edition
+ *
+ * Exercise 3-3: Write a function expand(s1,s2) that expands shorthand
+ * notations like 'a-z' in the string s1 into the equivalent complete list
+ * 'abc...xyz' in s2. Allow for letters of either case and digits, and be
+ * prepared to handle cases like 'a-b-c' and 'a-z0-9' and '-a-z'. Arrange that
+ * a leading or trailing '-' is taken literally.
+ *
+ * Answer: This one was fun. The trick is to target dashes, since they have the
+ * special behavior. From there, you decide what to do based on context. Once
+ * you've determined that the characters on both sides of the dash are letters
+ * OR numbers - no mixing - you store both values and put'em through a loop to
+ * output the range. There are a few edge cases to watch out for, but it's
+ * pretty simple to account for them.
+ *
+ * A limitation of my expand() is leading and trailing dashes are only output
+ * once. You can lead or trail with > 1 dash and it will truncate it to only 1.
+ * This could be seen as a bug or a feature. I haven't decided.
+ */
+
+void expand(char s1[], char s2[]) {
+	int i, j;
+	char start = 0;
+	char end = 0;
+	for (i = 0, j = 0; s1[i] != '\0'; i++) {
+		if (s1[i] == '-') {
+			if ((isalpha(s1[i - 1]) && isalpha(s1[i + 1])) || (isdigit(s1[i - 1]) && isdigit(s1[i + 1]))) {
+				start = s1[i - 1];
+				end = s1[i + 1];
+				start = tolower(start);
+				end = tolower(end);
+				if (start == s2[j - 1]) {
+					start++;
+				}
+				while (start <= end) {
+					s2[j++] = start++;
+				}
+			} else {
+				if (i == 0 || s1[i + 1] == '\0') {
+					s2[j++] = '-';
+				}
+			}
+		}
+	}
+	s2[j] = '\0';
+}
+
+int main() {
+	char foo[21] = "A-Z";
+	char bar[21] = "0-9";
+	char baz[21] = "A-g-N";
+	char fob[21] = "2-5-9";
+	char woot[21] = "-a-f0-36-9b-h-";
+	char derp[21] = "a-28-t";
+	char dest[61] = "";
+
+	printf("%26s\n", "INPUT |  OUTPUT");
+	printf("%27s\n", "-----------------");
+
+	expand(foo, dest);
+	printf("%16s => %s\n", foo, dest);
+
+	expand(bar, dest);
+	printf("%16s => %s\n", bar, dest);
+
+	expand(baz, dest);
+	printf("%16s => %s\n", baz, dest);
+
+	expand(fob, dest);
+	printf("%16s => %s\n", fob, dest);
+
+	expand(woot, dest);
+	printf("%16s => %s\n", woot, dest);
+
+	expand(derp, dest);
+	printf("%16s => %s\n", derp, dest);
+
+	return 0;
+}