How do I write a simple regular expression pattern matching function in C or C++?

Question

This is a question in my paper test today, the function signature is

int is_match(char* pattern,char* string)

The pattern is limited to only A

Answer 1

Didn't test this, actually code it, or debug it, but this might get you a start...

for each character in the pattern
  if pattern character after the current one is *
    // enter * state
    while current character from target == current pattern char, and not at end
      get next character from target
    skip a char from the pattern
  else if pattern character after the current one is ?
    // enter ? state
    if current character from target == current pattern char
      get next char from target
    skip a char from the pattern
  else
    // enter character state
    if current character from target == current pattern character
      get next character from target
    else
      return false
return true

Answer 2

Simple recursive implementation. It's slow but easy to understand:

int is_match(char *pattern, char *string)
{
    if (!pattern[0]) {
        return !string[0];
    } else if (pattern[1] == '?') {
        return (pattern[0] == string[0] && is_match(pattern+2, string+1))
            || is_match(pattern+2, string);
    } else if (pattern[1] == '*') {
        size_t i;
        for (i=0; string[i] == pattern[0]; i++)
            if (is_match(pattern+2, string+i)) return 1;
        return 0;
    } else {
        return pattern[0] == string[0] && is_match(pattern+1, string+1);
    }
}

Hope I got it all right.

Answer 3

See This Question for a solution you can not submit. See this paper for a description of how to implement a more readable one.