You will need to order these correctly (e.g.
An approach here is to have a list of regular expressions that match the token and associate that with the token type. Using a big regular expression is problematic as you noted, as you cannot get the token type.
numbers, letters, minus and less than) that you can define as a 256 item lookup table. You can also split the characters into different classes (e.g. This can be easier to read and simpler to write. The alternative approach is to switch on the first character and then read the rest of the characters in that token as part of that case statement. Here, you will have a token_type tokens array that you can then do token = tokens to get the token. With this approach, the end state can be used to determine the token type. Transition tables are like the switch statements: the rows define the states, the columns define the data values and the cells define the next state to transition to (with something like -1 to signal to stop processing). Automatic lexical analyzer generation How do Lex and similar tools do their job Lex translates regular expressions into transition diagrams. using transition tables instead of switch statements). The upside is that it is easier to refactor to take advantage of finite state machines (e.g. which has states 0 to 3, where state 0 is the initial state and state 3 is an accept state that indicates a possible end of the pattern. LEX converts each set of regular expressions into a Deterministic FSA (DFSA) e.g. The downside to this is that it can get complex to maintain, especially if there are many states to process each token. Lexers - Finite State Automata (FSA) Lexers are also known as scanners. This is an implementation of a finite state machine to process the tokens. the inner switch statement is switching on the character that was read.the outer switch statement is switching on the state.Writing a finite state machine (FSM) lexer by hand, you loop over the characters in your input and process that with two levels of switch statements:
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