实现正则表达式的.和*

失败的经历：

1.想用循环来实现，发现基本不可能啊。

2.想用NFA->DFA实现，还是算了吧，又不是真的实现正则表达式引擎。

建议先自己实现，我这边有testCase可以测试，一边调试一边找错，会发现最后的实现很不错。啥都不说了，上代码。

本题目我最招是从coding_interview上看到的，这本书也很是不错。

// How do you implement a function to match regular expressions with ‘.’ and ‘*’ in patterns? The
// character ‘.’ in a pattern matches a single character, and ‘*’ matches zero or any number of characters preceding
// it. Matching means that a string fully matches the pattern where all characters in a string match the whole pattern.
// For example, the string “aaa” matches the pattern “a.a” and the pattern “ab*ac*a”. However, it does not match
// the pattern “aa.a” nor “ab*a”.


#include <stdio.h>
#include <assert.h>


bool match(char* myString, char* pattern){


if(myString == NULL || pattern == NULL){
return false;
}


if(*myString != '\0' && *pattern != '\0'){
if(pattern[1] == '*'){
// this recursive can't be replaced by loop, they work together and coordinate complicatedly
if(*myString == *pattern || *pattern == '.'){
return match(myString, pattern + 2)//skip * 
|| match(myString + 1, pattern + 2)//eat a ch and skip *
|| match(myString + 1, pattern)//stay at current parrtern
;
}
else{
return match(myString, pattern + 2);//skip * 
}
}
else{
if(*myString == *pattern || *pattern == '.'){
return match(myString + 1, pattern + 1);
}
else{
return false;
}
 
}
}//for
if(*myString != '\0' || *pattern != '\0'){
return false;
}
return true;
}


//test case 0. NULL , a.a
//test case 0. NULL , NULL
//test case 0. aaa , NULL
//test case 1. aaa , a.a
//test case 1. aaa , a.*a
//test case 2. aaa , ab*ac*a
//test case 3. aaa , aa.a
//test case 4. aaa , ab*a
//test case 4. aaa , .
//test case 4. aaa , * //invalid
//test case 4. aaa , .*
//test case 4. aaa , a.*
//test case 4. aaa , a*


void testCase0(){

bool 
result = match(NULL, "a.a");
assert(!result);
result = match(NULL, NULL);
assert(!result);
result = match("aaa", NULL);
assert(!result);
result = match("aaa", "a.a");
assert(result);
result = match("aaa", "a.*a");
assert(result);
result = match("aaa", "ab*ac*a");
assert(result);
result = match("aaa", "aa.a");
assert(!result);
result = match("aaa", "ab*a");
assert(!result);
result = match("aaa", ".");
assert(!result);
result = match("aaa", "*");
assert(!result);
result = match("aaa", ".*");
assert(result);
result = match("aaa", "a.*");
assert(result);
result = match("aaa", "a*");
assert(result);
}
int main(){
testCase0();
}