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Regular Expression Matching in JavaScript
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
Updated on: 11-Dec-2020
174 Views
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