Return an error if a POSIX charclass is specified outside of brackets

compile.go

@@ -481,6 +481,16 @@ func shuntingYard(re string, flags ...ReFlag) ([]postfixNode, error) {
 			if i >= len(re_postfix) {
 				return nil, fmt.Errorf("Opening bracket with no closing bracket.")
 			}
+
+			// Check if a POSIX character class was specified ouside a bracket. This is an error.
+			// Eg. [:digit:] should lead to an error, telling the user that the right syntax is [[:digit:]]
+			if re_postfix[i] == ':' {
+				posixClassPresent, _ := getPOSIXClass(re_postfix[i+1:])
+				if posixClassPresent {
+					return nil, fmt.Errorf("The syntax for POSIX character classes is [[:digit:]], not [:digit:]")
+				}
+			}
+
 			var invertMatch bool
 			if re_postfix[i] == '^' {
 				invertMatch = true