C Program to Implement Regular Expression Matching

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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.

  1. /*
  2.  * C Program to Implements Regular Expression Matching
  3.  */
  4. #include <stdio.h>
  5. #include <string.h>
  6. #define MATCH printf("\nThe Text Matches The Regular Expression");
  7. #define NOTMATCH printf("\nThe Text Doesn't match the Regular Expression");
  8.  
  9. char reg[20], text[20];
  10.  
  11. int main()
  12. {
  13.     int i, rlen, tlen, f = 0;
  14.     char ans;
  15.  
  16.     do {
  17.             printf("\nEnter the Regular Expression\n");    
  18.             scanf(" %[^\n]s", reg);
  19.             for (rlen = 0; reg[rlen] != '\0';rlen++);
  20.             printf("\nEnter the text\n");
  21.             scanf(" %[^\n]s", text);
  22.             for (tlen = 0;text[tlen] != '\0' ; tlen++);
  23.             if (reg[0] == '*')
  24.             {
  25.                 printf("\nInvalid regular expression");
  26.             }
  27.             /*
  28.              *If the regular expression starts with Alphabet
  29.              */
  30.             if ((reg[0] >= 65 && reg[0] <= 90) || (reg[0] >= 97 && reg[0] <=122))
  31.             {
  32.                 if (reg[0] == text [0])
  33.                 {
  34.                     switch (reg[1]) 
  35.                     {
  36.                     case '.' :
  37.                         switch (reg[2])
  38.                         {
  39.                         case '*':
  40.                             if (tlen != 1)
  41.                             {
  42.                                 if (reg[3] == text[tlen-1])
  43.                                 {
  44.                                     MATCH;
  45.                                 }
  46.                                 else
  47.                                 {
  48.                                     NOTMATCH;
  49.                                 }
  50.                             }
  51.                             else
  52.                             {
  53.                                 NOTMATCH;
  54.                             }
  55.                             break;
  56.                         case '+':
  57.                             if (text[1] != reg[3])
  58.                             {
  59.                                 if (reg[3] == text[tlen - 1])
  60.                                 {
  61.                                     MATCH;
  62.                                 }
  63.                                 else
  64.                                 {
  65.                                     NOTMATCH;
  66.                                 }
  67.                             }
  68.                             break;
  69.                         case '?':
  70.                             if (text[1] == reg[3] || text[2] == reg[3])
  71.                             {
  72.                                 if (text[1] == reg[3] || text[2] == reg[3])
  73.                                 {
  74.                                     MATCH;
  75.                                 }
  76.                                 else
  77.                                 {
  78.                                     NOTMATCH;
  79.                                 }
  80.                             }
  81.                             else
  82.                             {
  83.                                 NOTMATCH;
  84.                             }
  85.                              break;
  86.                             }
  87.                             break;
  88.                         case '*':
  89.                             if (reg[rlen-1] == text[tlen-1])
  90.                             {
  91.                                 for (i = 0;i <= tlen-2;i++)
  92.                                 {
  93.                                     if(text[i] == reg[0])
  94.                                     {
  95.                                         f = 1;        
  96.                                     }
  97.                                     else 
  98.                                     {
  99.                                         f = 0;
  100.                                     }
  101.                                 }
  102.                                 if ( f == 1)
  103.                                 {
  104.                                     MATCH;
  105.                                 }
  106.                                 else
  107.                                 {
  108.                                     NOTMATCH;
  109.                                 }
  110.                             }
  111.                             else
  112.                             {
  113.                                 NOTMATCH;
  114.                             }
  115.                             break;
  116.                     case '+' :
  117.                         if (tlen <= 2)
  118.                         {    
  119.                             NOTMATCH;
  120.                         }
  121.                         else if (reg[rlen-1] == text[tlen-1])
  122.                         {
  123.                             for (i = 0;i < tlen-2;i++)
  124.                             {
  125.                                 if (text[i] == reg[0])
  126.                                 {
  127.                                     f = 1;
  128.                                 }
  129.                                 else
  130.                                 {
  131.                                     f = 0;
  132.                                 }
  133.                             }
  134.  
  135.                             if (f == 1)
  136.                             {
  137.                                 MATCH;
  138.                             }
  139.                             else 
  140.                             {
  141.                                 NOTMATCH;
  142.                             }
  143.                         }
  144.                             break;
  145.                     case '?':
  146.                         if (reg[rlen -1] == text[tlen-1])
  147.                         {
  148.                             MATCH;
  149.                         }
  150.                         else
  151.                         {
  152.                             NOTMATCH;
  153.                         }
  154.                     break;
  155.                 } 
  156.  
  157.             }
  158.             else
  159.                 printf("Does not match");             
  160.         }
  161.         /*
  162.          *If Regular Expression starts with '^'
  163.          */
  164.         else if (reg[0] == '^')
  165.         {
  166.             if (reg[1] == text[0])
  167.             {
  168.                 MATCH;
  169.             }
  170.             else
  171.             {
  172.                 NOTMATCH;
  173.             }
  174.         }
  175.         /*
  176.          *If Regular Expression Ends with '$'
  177.          */
  178.         else if (reg[rlen-1] == '$')
  179.         {
  180.             if (reg[rlen-2] == text[rlen-1])
  181.             {
  182.                 MATCH;
  183.             }
  184.             else
  185.             { 
  186.                 NOTMATCH;
  187.             }
  188.         }
  189.  
  190.         else
  191.             printf("Not Implemented");
  192.         printf("\nDo you want to continue?(Y/N)");
  193.         scanf(" %c", &ans);
  194.     } while (ans == 'Y' || ans == 'y');
  195. }

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$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

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Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He is Linux Kernel Developer & SAN Architect and is passionate about competency developments in these areas. He lives in Bangalore and delivers focused training sessions to IT professionals in Linux Kernel, Linux Debugging, Linux Device Drivers, Linux Networking, Linux Storage, Advanced C Programming, SAN Storage Technologies, SCSI Internals & Storage Protocols such as iSCSI & Fiber Channel. Stay connected with him @ LinkedIn