diff --git a/backend/gs1.c b/backend/gs1.c
index d9794883..7296cb26 100644
--- a/backend/gs1.c
+++ b/backend/gs1.c
@@ -26,36 +26,70 @@
 int gs1_verify(struct zint_symbol *symbol, unsigned char source[], char reduced[])
 {
 	int i, j, last_ai, ai_latch;
-	char ai_string[4];
+	char ai_string[6];
+	int bracket_level, max_bracket_level, ai_length, max_ai_length, min_ai_length;
 	
 	/* Detect extended ASCII characters */
-	for(i = 0; i <  sourcelen; i++) {
+	for(i = 0; i <  ustrlen(source); i++) {
 		if(source[i] >=128) {
 			strcpy(symbol->errtxt, "Extended ASCII characters are not supported by GS1");
 			return ERROR_INVALID_DATA;
 		}
 	}
 	
-	/* Start with basic tests */
 	if(source[0] != '[') {
 		strcpy(symbol->errtxt, "Data does not start with an AI");
 		return ERROR_INVALID_DATA;
 	}
 	
-	for(i = 0; i < ustrlen(source) - 1; i++) {
-		if((source[i] == '[') && (source[i + 1] == '[')) {
-			/* Can't have nested brackets - Quit */
-			strcpy(symbol->errtxt, "Nested AI detected (two or more open brackets)");
-			return ERROR_INVALID_DATA;
-		}
+	/* Check the position of the brackets */
+	bracket_level = 0;
+	max_bracket_level = 0;
+	ai_length = 0;
+	max_ai_length = 0;
+	min_ai_length = 5;
+	j = 0;
+	for(i = 0; i < ustrlen(source); i++) {
+		ai_length += j;
+		if(source[i] == '[') { bracket_level++; j = 1; }
+		if(source[i] == ']') {
+			bracket_level--;
+			if(ai_length < min_ai_length) { min_ai_length = ai_length; }
+			j = 0;
+			ai_length = 0; }
+		if(bracket_level > max_bracket_level) { max_bracket_level = bracket_level; }
+		if(ai_length > max_ai_length) { max_ai_length = ai_length; }
+	}
+	min_ai_length--;
+	
+	if(bracket_level != 0) {
+		/* Not all brackets are closed */
+		strcpy(symbol->errtxt, "Malformed AI in input data (brackets don\'t match)");
+		return ERROR_INVALID_DATA;
 	}
 	
-	for(i = 0; i < ustrlen(source) - 1; i++) {
-		if((source[i] == ']') && (source[i + 1] == ']')) {
-			/* Can't have nested brackets - Quit */
-			strcpy(symbol->errtxt, "Nested AI detected (two or more close brackets)");
-			return ERROR_INVALID_DATA;
-		}
+	if(max_bracket_level > 1) {
+		/* Nested brackets */
+		strcpy(symbol->errtxt, "Found nested brackets in input data");
+		return ERROR_INVALID_DATA;
+	}
+	
+	if(max_ai_length > 4) {
+		/* AI is too long */
+		strcpy(symbol->errtxt, "Invalid AI in input data (AI too long)");
+		return ERROR_INVALID_DATA;
+	}
+	
+	if(min_ai_length <= 1) {
+		/* AI is too short */
+		strcpy(symbol->errtxt, "Invalid AI in input data (AI too short)");
+		return ERROR_INVALID_DATA;
+	}
+	
+	if(ai_latch == 1) {
+		/* Non-numeric data in AI */
+		strcpy(symbol->errtxt, "Invalid AI in input data (non-numeric characters in AI)");
+		return ERROR_INVALID_DATA;
 	}
 	
 	/* Resolve AI data - put resulting string in 'reduced' */