Regular Expression Matching

06/23/2016 Dynamic Programming Backtracking String

Question

Implement regular expression matching with support for '.' and '*'.

'.' Matches any single character.
'*' Matches zero or more of the preceding element.

The matching should cover the entire input string (not partial).

The function prototype should be:
bool isMatch(const char *s, const char *p)

Some examples:
isMatch("aa","a") → false
isMatch("aa","aa") → true
isMatch("aaa","aa") → false
isMatch("aa", "a*") → true
isMatch("aa", ".*") → true
isMatch("ab", ".*") → true
isMatch("aab", "c*a*b") → true


Solution

Result: Accepted Time: 20ms

Here should be some explanations.

bool isMatch(char* s, char* p) {
if(s == NULL || p == NULL)
return false;
if(*p==0)
return *s == 0;
if(*(p+1) == '*')
do{
if(isMatch(s,p+2))
return true;
}while(*s && (*p == *(s++) || *p == '.'));
if(*p == *s || (*p == '.' && *s!=0))
return isMatch(++s,++p);
return false;
}
/*
"bbbba"
".*a*a"

"ab"
".*c"

*/


Complexity Analytics

• Time Complexity: $O(n)$
• Space Complexity: $O(1)$