 Forth has no explicit grammar . The interpreter reads a line of input from the user input device , which is then parsed for a word using spaces as a <unk> ; some systems recognise additional whitespace characters . When the interpreter finds a word , it looks the word up in the dictionary . If the word is found , the interpreter executes the code associated with the word , and then returns to parse the rest of the input stream . If the word isn 't found , the word is assumed to be a number and an attempt is made to convert it into a number and push it on the stack ; if successful , the interpreter continues parsing the input stream . Otherwise , if both the lookup and the number conversion fail , the interpreter prints the word followed by an error message indicating the word is not recognised , flushes the input stream , and waits for new user input . 
