PHP preg_match部分字符串有效

Question

For example, given the following function definition:

function match($subject, $pattern, $offset);

These tests should succeed:

$subject = "23 is a number, so is 10";

match($subject, '[0-9]+', 0) == '23';
match($subject, '[0-9]+', 3) == false;
match($subject, '[0-9]+', 6) == false;
match($subject, '[0-9]+', 8) == false;

match($subject, '[a-z]+', 0) == false;
match($subject, '[a-z]+', 3) == 'is';
match($subject, '[a-z]+', 6) == 'a';
match($subject, '[a-z]+', 8) == 'number';

One possible way is to match a substring starting at $offset by using ^ :

function match($subject, $pattern, $offset) {

    if (preg_match("/^($pattern)/", substr($subject, offset), $matches)) {
        return $matches[1];
    }

    return false;
}

This would make a copy of a the string, which ofcourse isn't efficient for large strings.

Another possible way to implement match would be:

function match($subject, $pattern, $offset) {

    if (preg_match("/($pattern)/", $subject, $matches, PREG_OFFSET_CAPTURE, $offset)) {
        if ($matches[1][1] == $offset) {
            return $matches[1][0];
        }
    }

    return false;
}

But this would keep trying to match even if the first character mismatches.

Question : How can I match a part of a string efficiently?

Or maybe better, is it possible to assert the offset position? Like ^ would assert the start of the string.

Answer 1

You can try something like this:

function match($subject, $pattern, $offset) {
    if (preg_match('/^.{' . $offset . '}\K' . $pattern . '/us', $subject, $match))
        return $match[0];
    return false;
}

---

Better! You can use the \\G anchor that matches at the offset:

function match($subject, $pattern, $offset) {
    if (preg_match('/\G' . $pattern . '/us', $subject, $match, 0, $offset))
        return $match[0];
    return false;
}