simple_re_match.c

#include <stdio.h>

#include "simple_re_match.h"

/*
Regular expresions:
    c    matches any literal character c
    .    matches any single character
    ^    matches the beginning of the input string
    $    matches the end of the input string
    *    matches zero or more occurrences of the previous character
    +    matches one or more occurrences of the previous character
*/

/*
 @brief search for regexp anywhere in the text and return the index that matches
 @param regular expression string
 @param string to perform the regex on
 */
int
match(char *regexp, char *text) 
{
    if (regexp[0] == '^')
        return matchhere(regexp+1, text);
    do { //should check if the string is empty
        if (matchhere(regexp, text))
            return 1;
    }while(*text++ != '\0');
    return 0;
}

int
matchhere(char *regexp, char *text)
{
    if (regexp[0] == '\0')
        return 1;
    if (regexp[1] == '*')
        return matchstar(regexp[0], regexp+2, text);
    if (regexp[1] == '+')
        return matchplus(regexp[0], regexp+2, text);
    if (regexp[0] == '\0' && regexp[1] == '\0')
        return *text == '\0';
    if (*text != '\0' && (regexp[0] == '.' || regexp[0] == *text))
        return matchhere(regexp + 1, text + 1);
    return 0;
}

/* matchstar: leftmost longest search for c*regexp */
int 
matchstar(int c, char *regexp, char *text)
{
    char *t;
    for (t = text; *t != '\0' && (*t == c || c == '.'); t++)
        ;
    do {        /* * matches zero or more */
        if (matchhere(regexp, t))
            return 1;
    } while (t-- > text);
    return 0;
}

int
matchplus(int c, char *regexp, char *text)
{
    int i = 0;
    do { //a matches one or more instances
        if (matchhere(regexp, text))
            if (i == 0)
                i++;
            else
                return 1;
    } while (*text != '\0' && (*text++ == c || c == '.'));
    return 0;
}

int main(){
    char *regex = "c+";
    char *text = "caglar";
    if(match(regex, text))
        printf("Ok!\n");
    else 
        printf("No!\n");
    return 0;
}