Suppose we are given an input string str and a pattern p, we are required to implement regular expression matching with support for. and *.
The functions of these symbols should be −
. --> Matches any single character.
* --> Matches zero or more of the preceding elements.
The matching should cover the entire input string (not partial).
Note
str could be empty and contains only lowercase letters a-z.
p could be empty and contains only lowercase letters a-z, and characters like. or *.
For example −
If the input is −
const str = 'aa'; const p = 'a';
Then the output should be false because a does not match the entire string aa.
Example
Following is the code −
const regexMatching = (str, p) => { const ZERO_OR_MORE_CHARS = '*'; const ANY_CHAR = '.'; const match = Array(str.length + 1).fill(null).map(() => { return Array(p.length + 1).fill(null); }); match[0][0] = true; for (let col = 1; col <= p.length; col += 1) { const patternIndex = col - 1; if (p[patternIndex] === ZERO_OR_MORE_CHARS) { match[0][col] = match[0][col - 2]; } else { match[0][col] = false; } } for (let row = 1; row <= str.length; row += 1) { match[row][0] = false; } for (let row = 1; row <= str.length; row += 1) { for (let col = 1; col <= p.length; col += 1) { const stringIndex = row - 1; const patternIndex = col - 1; if (p[patternIndex] === ZERO_OR_MORE_CHARS) { if (match[row][col - 2] === true) { match[row][col] = true; } else if ( ( p[patternIndex - 1] === str[stringIndex] || p[patternIndex - 1] === ANY_CHAR ) && match[row - 1][col] === true ) { match[row][col] = true; } else { match[row][col] = false; } } else if ( p[patternIndex] === str[stringIndex] || p[patternIndex] === ANY_CHAR ) { match[row][col] = match[row - 1][col - 1]; } else { match[row][col] = false; } } } return match[str.length][p.length]; }; console.log(regexMatching('aab', 'c*a*b'));
Output
Following is the output on console −
true