How to write regular expressions in C
Educative Answers TeamA 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:
- A pointer to the memory location where the pattern to be matched is stored.
- A pointer to a pattern of the string type.
- 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(®ex,"[: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:
- A precompiled pattern
- A string in which the pattern needs to be searched for.
- Information regarding the location of matches.
- Information regarding the location of matches.
- 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(®ex,"ice",0);
return_value = regexec(®ex, "icecream", 0, NULL, 0);
return_value2 = regcomp(®ex,"ice",0);
return_value2 = regexec(®ex, "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;
}Enter the input below
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