Regular Expression Matching in C

This C Program implements regular expression matching.

Here is source code of the C Program to implement regular expression matching. The C program is successfully compiled and run on a Linux system. The program output is also shown below.

```
/*
```

 * C Program to Implements Regular Expression Matching

```
 */
```
```
#include <stdio.h>
```
```
#include <string.h>
```

#define MATCH printf("\nThe Text Matches The Regular Expression");

#define NOTMATCH printf("\nThe Text Doesn't match the Regular Expression");

```
 
```
```
char reg[20], text[20];
```
```
 
```
```
int main()
```
```
{
```
```
    int i, rlen, tlen, f = 0;
```
```
    char ans;
```
```
 
```
```
    do {
```

            printf("\nEnter the Regular Expression\n");

```
            scanf(" %[^\n]s", reg);
```

            for (rlen = 0; reg[rlen] != '\0';rlen++);

            printf("\nEnter the text\n");

```
            scanf(" %[^\n]s", text);
```

            for (tlen = 0;text[tlen] != '\0' ; tlen++);

```
            if (reg[0] == '*')
```
```
            {
```

                printf("\nInvalid 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("\nDo you want to continue?(Y/N)");

```
        scanf(" %c", &ans);
```
```
    } while (ans == 'Y' || ans == 'y');
```
```
}
```

$gcc -o regex regular.c
$ ./regex
 
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