Depends on my personal library (currently private).

regularexpressions.c

#include <stdio.h>
#define AB_WIN
#include "ab.h"

void MatchString(char *Match, const char *String, char *Result) {
	int StringIndex = 0;
	int MatchLength = 0;
	int ResultLength = 0;
	bool Class = false;
	bool JustRestart = false;
	while (StringIndex < ab_strlen(String) && String[StringIndex]) {
		bool Replace = false;
		// Literal
		if (String[StringIndex] == *Match) Replace = true;
		if (*Match == '.') Replace = true;
		if (*Match == '\\') {
			*Match++;
			MatchLength++;
			Class = true;
		}
		// Classes
		if (Class) {
			// Alpha
			if (*Match == 'w' && ab_isalpha(String[StringIndex])) Replace = true;
			// Numberic
			if (*Match == 'd' && ab_isnum(String[StringIndex])) Replace = true;
			// Alphanumeric
			if (*Match == 'a' && ab_isalphanum(String[StringIndex])) Replace = true;
		}
		// Quantifiers
		if (Match[1] == '?') {
			if (Replace) { // Match, optional
				*Match++;
				MatchLength++;
			} else { // No match, optional.
				*Match++;
				*Match++;
				MatchLength++;
				MatchLength++;
				// StringIndex--;
				continue;
			}
		}
		if (Replace) {
			*Result++ = String[StringIndex];
			ResultLength++;
			// printf("%s ", Result);
			*Match++;
			MatchLength++;
			Class = false;
			JustRestart = false;
		} else {
			while (--MatchLength >= 0) *Match--;
			while (--ResultLength >= 0) *Result--;
			StringIndex = 0;
			if (JustRestart) { 
				*Result--; 
				break; 
			};
			JustRestart = true;
		}
		StringIndex++;
	}
	*Result = '\0';
}

int main(int argc, char *argv[]) {
	// Do not buffer stdout.
	setvbuf(stdout, NULL, _IONBF, 0);

	char Match[] = "\\worl?d";
	// char Match[] = "\\w\\d";
	char Result[10] = { 0 };
	MatchString(Match, "World", Result);
	printf("%s\n", Result);
	return 0;
}