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Last Updated : 01 Nov, 2023
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Prerequisite: How to write Regular Expressions?
A regular expression is a sequence of characters that is used to search pattern. It is mainly used for pattern matching with strings, or string matching, etc. They are a generalized way to match patterns with sequences of characters. It is used in every programming language like C++, Java, and Python.
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. |
Creation of Regular Expression
For compiling or creating the regular expression regcomp() function is used. It takes three arguments:
Syntax:
regcomp(®ex, expression, flag)
where,
See Also
C Library - - regex is a pointer to a memory location where expression is matched and stored.
- expression is a string type
- flag to specify the type of compilation
Return Value: This returns the value as shown below:
- 0: when successful compilation is done.
- Error_code: When there is unsuccessful compilation of the expression.
Below is the illustration of the regcomp() function:
C
// C program for illustration of regcomp()
#include <regex.h>
#include <stdio.h>
// Driver Code
int main()
{
// Variable to create regex
regex_t reegex;
// Variable to store the return
// value after creation of regex
int value;
// Function call to create regex
value = regcomp( &reegex, "[:word:]", 0);
// If compilation is successful
if (value == 0) {
printf("RegEx compiled successfully.");
}
// Else for Compilation error
else {
printf("Compilation error.");
}
return 0;
}
Output
RegEx compiled successfully.
Time complexity : O(n)
Auxiliary Space : O(1)where n is the length of the pattern
Matching of Pattern using Regular Expression
The regexec() function is used to match a string against a pattern. 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.
- Flags to specify a change in the matching behavior.
Syntax:
regexec(®ex, expression, 0, NULL, 0);
where, regex = precompiled pattern,
expression = pattern to be match in regex,
NULL = Information regarding location of the matches.
flag = to specify the change in matching behaviour
Return Value: This returns the value as shown below:
- 0: If there is a match.
- REG_NOMATCH: If there is no match.
Below is the illustration of the regexec() function:
C
// C program to illustrate the regexec() function
#include <regex.h>
#include <stdio.h>
// Function to print the result
void print_result(int value)
{
// If pattern found
if (value == 0) {
printf("Pattern found.\n");
}
// If pattern not found
else if (value == REG_NOMATCH) {
printf("Pattern not found.\n");
}
// If error occurred during Pattern
// matching
else {
printf("An error occurred.\n");
}
}
// Driver Code
int main()
{
// Variable to store initial regex()
regex_t reegex;
// Variable for return type
int value;
int value2;
// Creation of regEx
value = regcomp( &reegex, "Welcome to GfG", 0);
// Comparing pattern "GeeksforGeeks" with
// string in reg
value = regexec( &reegex, "GeeksforGeeks",
0, NULL, 0);
// Creation of regEx
value2 = regcomp( &reegex, "GeeksforGeeks", 0);
// Comparing pattern "Geeks"
// with string in reg
value2 = regexec( &reegex, "GeeksforGeeks",
0, NULL, 0);
// Print the results
print_result(value);
print_result(value2);
return 0;
}
Output
Pattern not found.Pattern found.
Time complexity : O(n)
Auxiliary Space : O(1)where n is the length of the pattern
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