A **regular expression** is a sequence of characters used to match a pattern to a string. The expression can be used for searching text and validating input.

Remember, a regular expression is not the property of a particular language.

POSIX is a well-known library used for regular expressions in C.

# Patterns in the POSIX library

| **Expression**   | **Description**  | 
|----------:|:-----------:|:--------------|
| []   | Used to find any of the characters or numbers specified between the brackets.     | 
| [:number:]   | Used to find any digit.     |
| [:lower:]   | Used to find lowercase alphabets.     |
| [:word:]   | Used to find letters numbers and underscores.     |
   

# Creating a regular expression

The `regcomp()` function is used to compile a regular expression. According to the official documentation, it takes three arguments:
1. A pointer to the memory location where the pattern to be matched is stored.
2. A pointer to a pattern of the string type.
3. A flag to specify the type of compilation.

It returns a 0 upon successful compilation,​ and an error code otherwise.

#include<stdio.h>
// Importing the POSIX regex library
#include <regex.h> 
int main() {
    regex_t regex;
    int return_value;
    return_value = regcomp(&regex,"[:lower:]",0);
    if (return_value == 0){
        printf("Regular expression compiled successfully.");
    }
    else{
        printf("Compilation error.");
    }
    return 0;
}

# Matching a pattern

The `regexec()` function is used to match a string against a pattern. According to the official documentation, it takes in five arguments:
1. A precompiled pattern
2. A string in which the pattern needs to be searched for.
3. Information regarding the location of matches.
4. Information regarding the location of matches.
5. Flags to specify a change in the matching behavior.

It returns a 0 if there is a match, and a `REG_NOMATCH` error otherwise.

#include<stdio.h>
// Importing the POSIX regex library
#include <regex.h> 
void print_result(int return_value){
  if (return_value == 0){
    printf("Pattern found.\n");
  }
  else if (return_value == REG_NOMATCH){
    printf("Pattern not found.\n");
  }
  else{
    printf("An error occured.\n");
  }
}
int main() {
  regex_t regex;
  int return_value;
  int return_value2;
  return_value = regcomp(&regex,"ice",0);
  return_value = regexec(&regex, "icecream", 0, NULL, 0);
  return_value2 = regcomp(&regex,"ice",0);
  return_value2 = regexec(&regex, "frozen yoghurt", 0, NULL, 0);
  print_result(return_value);
  print_result(return_value2);
  return 0;
}

# Using a regular expression with `scanf()`

Using a regular expression to accept only letters as input.

#include<stdio.h>
#include<stdlib.h>
int main() {
    char name[15];
    // Taking a name as an input.
    // name can only include alphabets
    scanf("%[a-zA-Z]",name);
    printf("%s",name);
    return 0;
}

How to write regular expressions in C

Regular expressions in C match patterns using the POSIX library. Key functions include regcomp() for compiling and regexec() for matching patterns.

Expression	Description
[]	Used to find any of the characters or numbers specified between the brackets.
[:number:]	Used to find any digit.
[:lower:]	Used to find lowercase alphabets.
[:word:]	Used to find letters numbers and underscores.

How to write regular expressions in C

Patterns in the POSIX library

Creating a regular expression

Matching a pattern

Using a regular expression with scanf()

Using a regular expression with `scanf()`