/*
* C Program to Implements Regular Expression Matching
*/
#include <stdio.h>
#include <string.h>
#define MATCH printf("
The Text Matches The Regular Expression");
#define NOTMATCH printf("
The Text Doesn't match the Regular Expression");

char reg[20], text[20];
int main()
{
    int i, rlen, tlen, f = 0;
    char ans;
    do
        {
            printf("
                   Enter the Regular Expression
                   ");
                   scanf(" %[^
                        ]s", reg);
                   for (rlen = 0; reg[rlen] != ''; rlen++);
                   printf("
                          Enter the text
                          ");
                          scanf(" %[^
                               ]s", text);
                          for (tlen = 0; text[tlen] != '' ; tlen++);
                              if (reg[0] == '*')
                    {
                        printf("
                               Invalid regular expression");
                    }
        /*
         *If the regular expression starts with Alphabet
         */
        if ((reg[0] >= 65 && reg[0] <= 90) || (reg[0] >= 97 && reg[0] <=122))
            {
                if (reg[0] == text [0])
                        {
                            switch (reg[1])
                                {
                                case '.' :
                                    switch (reg[2])
                                        {
                                        case '*':
                                            if (tlen != 1)
                                                {
                                                    if (reg[3] == text[tlen-1])
                                                        {
                                                            MATCH;
                                                        }
                                                    else
                                                        {
                                                            NOTMATCH;
                                                        }
                                                }
                                            else
                                                {
                                                    NOTMATCH;
                                                }
                                            break;
                                        case '+':
                                            if (text[1] != reg[3])
                                                {
                                                    if (reg[3] == text[tlen - 1])
                                                        {
                                                            MATCH;
                                                        }
                                                    else
                                                        {
                                                            NOTMATCH;
                                                        }
                                                }
                                            break;
                                        case '?':
                                            if (text[1] == reg[3] || text[2] == reg[3])
                                                {
                                                    if (text[1] == reg[3] || text[2] == reg[3])
                                                        {
                                                            MATCH;
                                                        }
                                                    else
                                                        {
                                                            NOTMATCH;
                                                        }
                                                }
                                            else
                                                {
                                                    NOTMATCH;
                                                }
                                            break;
                                        }
                                    break;
                                case '*':
                                    if (reg[rlen-1] == text[tlen-1])
                                        {
                                            for (i = 0; i <= tlen-2; i++)
                                                {
                                                    if(text[i] == reg[0])
                                                        {
                                                            f = 1;
                                                        }
                                                    else
                                                        {
                                                            f = 0;
                                                        }
                                                }
                                            if ( f == 1)
                                                {
                                                    MATCH;
                                                }
                                            else
                                                {
                                                    NOTMATCH;
                                                }
                                        }
                                    else
                                        {
                                            NOTMATCH;
                                        }
                                    break;
                                case '+' :
                                    if (tlen <= 2)
                                        {
                                            NOTMATCH;
                                        }
                                    else if (reg[rlen-1] == text[tlen-1])
                                        {
                                            for (i = 0; i < tlen-2; i++)
                                                {
                                                    if (text[i] == reg[0])
                                                        {
                                                            f = 1;
                                                        }
                                                    else
                                                        {
                                                            f = 0;
                                                        }
                                                }
                                            if (f == 1)
                                                {
                                                    MATCH;
                                                }
                                            else
                                                {
                                                    NOTMATCH;
                                                }
                                        }
                                    break;
                                case '?':
                                    if (reg[rlen -1] == text[tlen-1])
                                        {
                                            MATCH;
                                        }
                                    else
                                        {
                                            NOTMATCH;
                                        }
                                    break;
                                }
                        }
                    else
                        printf("Does not match");
                }
            /*
             *If Regular Expression starts with '^'
             */
            else if (reg[0] == '^')
            {
                if (reg[1] == text[0])
                        {
                            MATCH;
                        }
                    else
                        {
                            NOTMATCH;
                        }
                }
            /*
             *If Regular Expression Ends with '$'
             */
            else if (reg[rlen-1] == '$')
            {
                if (reg[rlen-2] == text[rlen-1])
                        {
                            MATCH;
                        }
                    else
                        {
                            NOTMATCH;
                        }
                }
            else
                printf("Not Implemented");
                printf("
                       Do you want to continue?(Y/N)");
                       scanf(" %c", &ans);
            }
    while (ans == 'Y' || ans == 'y');
}


Enter the Regular Expression
C.*g

Enter the text
Cprogramming

The Text Matches The Regular Expression
Do you want to continue?(Y/N)y

Enter the Regular Expression
C*g

Enter the text
Cprogramming

The Text Doesn't match the Regular Expression
Do you want to continue?(Y/N)y

Enter the Regular Expression
C?.*g

Enter the text
Cprogramming

The Text Matches The Regular Expression
Do you want to continue?(Y/N)y

Enter the Regular Expression
C.?g

Enter the text
Cprogramming

The Text Doesn't match the Regular Expression
Do you want to continue?(Y/N)y

Enter the Regular Expression
C.+g

Enter the text
Cprogramming

The Text Matches The Regular Expression
Do you want to continue?(Y/N)y

Enter the Regular Expression
C+g

Enter the text
Cprogramming

The Text Doesn't match the Regular Expression
Do you want to continue?(Y/N)y

Enter the Regular Expression

^C.*

Enter the text
Cprogramming


The Text Matches The Regular Expression
Do you want to continue?(Y/N)y

Enter the Regular Expression
^p.*

Enter the text
Cprogramming

The Text Doesn't match the Regular Expression
Do you want to continue?(Y/N)y

Enter the Regular Expression
C.*g$

Enter the text
Cprogramming

The Text Matches The Regular Expression
Do you want to continue?(Y/N)y

Enter the Regular Expression
C.*n$

Enter the text
Cprogramming

The Text Doesn't match the Regular Expression
Do you want to continue?(Y/N)n