DFA (Recognizer) for valid Pascal identifiers

Problem – Implement a recognizer for pascal identifiers based on a DFA that accepts strings belonging to the definition of the language of the same.

Here is a regular definition for the set of Pascal identifiers that are defined as the set of strings of letters and digits beginning with a letter.

letter : A | B | . . . | Z | a | b | . . . | z
digit  : 0 | 1 | 2 | . . . | 9
ID : letter (letter | digit)* 

The regular expression ID is the pattern for the Pascal identifier token and defines letter and digit where a letter is a regular expression for the set of all upper-case and lowercase letters in the alphabet and digit is regular for the set of all decimal digits.

State diagram of the DFA

Working code for the recognizer:

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// C++ program to implement DFA based regonizer that accepts
// all strings which follow the language
// L = { letter (letter | digit)* }
  
#include <bits/stdc++.h>
#include <iostream>
using namespace std;
  
// dfa tells the number associated
// with the present state
int dfa;
  
// This function is for
// the starting state (zeroth) of DFA
void start(char c)
{
    if (isalpha(c))
        dfa = 1;
    else
  
// -1 is used to check for any invalid symbol
        dfa = -1; 
}
  
// This function is for the first state of DFA
void state1(char c)
{
    if (isalnum(c))
        dfa = 1;
    else
        dfa = -1;
}
  
bool DFA_for_ID(string token)
{
    dfa = 0;
    int i, len = token.length();
    for (i = 0; i < len; i++) {
        if (dfa == 0)
            start(token[i]);
        else if (dfa == 1)
            state1(token[i]);
        else
            return 0;
    }
    if (dfa == 1)
        return 1;
    else
        return 0;
}
  
// driver code
int main()
{
    string input = "Geeks for Geeks is 9ice platfo$m for every1  ";
  
// to seprate all the tokens by space in the string 
// and cheking for each token
    stringstream ss(input); 
    string token;
    while (ss >> token) {
        bool isValid = DFA_for_ID(token);
        if (isValid)
            cout << token << " : "
                 << "Valid" << endl;
        else
            cout << token << " : "
                 << "Invalid" << endl;
    }
    return 0;
}

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Output:

Geeks : Valid
for : Valid
Geeks : Valid
is : Valid
9ice : Invalid
platfo$m : Invalid
for : Valid
every1 : Valid


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