## Token, Pattern, Lexemes

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**Token:** Sequence of a character having a collective meaning is known as a token: token.

Typical tokens are,

1) Identifiers 2) keywords 3) operators 4) special symbols 5) constants

**Pattern:** The set of rules called pattern associated with a token.:

**Lexeme:** The sequence of character in a source program matched with a pattern for a token is:

called lexeme.

**Example: Token, Pattern, Lexemes.**

**total = sum + 12.5**

Tokens are : total (id),

= (relation)

Sum (id)

+ (operator)

12.5 (num)

Lexemes are: total, =, sum, +, 12.5

**2.Reorganization of Token**

- Here we address how to recognize token.
- We use the language generated by following grammar,

stmt → if expr then stmt

| if expr then stmt else stmt

|∈

expr → term relop term

| term

term → id | num

- Where the terminals if, then, else, relop, id and num generates the set of strings given by the following regular definitions,

if → if

then → then

relop → < | <= | = | <> | > | >=

letter → A | B | . . . | Z | a | b | . . . | z

digit → 0 | 1 | 2 | . . . | 9

id → letter (letter | digit)*

num → digit+ ()?(E(+/-)?digit+ )? - For this language, the lexical analyzer will recognize the keyword if, then, else, as well as the lexeme denoted by relop, id and num.
- num represents the unsigned integer and real numbers of Pascal.
- Lexical analyzer will isolate the next token in the input buffer and produces token and attribute value as an output.

**3.Transition diagram: Token, Pattern, Lexemes**

- A stylized flowchart is called transition diagram.
- Positions in a transition diagram are drawn as a circle and are called states.
- States are connected by arrows called edges.
- Edge leaving state have a label indicating the input character.
- Transition diagram for the token relop is shown in figure
- The transition diagram for an unsigned number is given in Fig. given

- Transition diagram for the token relop is shown in figure

**Finite automata (NFA & DFA) Automata: Token, Pattern, Lexemes**

- We compile a regular expression into a recognizer by constructing a generalized transition diagram called a finite automaton.
- A finite Automata or fin state machine is a 5-tuple(S,∑,S0,F,δ) wherefinite,δ)

S is finite set of states

∑ is finite alphabet of input symbol

S0 ∈ S (Initial state)

F (set of accepting states)

δ is a transition function

There are two types of finite automata

Deterministic finite automata (DFA) have for each state (circle in the diagram) exactly

- one edge leaving out for each symbol.ing Nondeterministic finite automata (NFA) are the other kind. There are no restrictions
- the edges leaving a state. There can be several with the same symbol as the label and somewhere edges can be labeled with ε.

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