Regex match numbers not followed by a hyphen

Question

I\'ve worked with regexes for years and never had this much trouble working out a regex. I am using PHP 7.2.2\'s preg_match() to return an array of matching numbers for parsing,

Answer 1

You may use

'~(\d+)x(\d++)(?!-)~'

It can also be written without a possessive quantifier as '~(\d+)x(\d+)(?![-\d])~' since the \d inside the lookahead will also forbid matching the second digit chunk partially.

Alternatively, additionally to the lookahead, you may use word boundaries:

'~\b(\d+)x(\d+)\b(?!-)~'

See the regex demo #1 and regex demo #2.

Details

• (\d+)x(\d++)(?!-) / (\d+)x(\d+)(?![-\d]) - matches and captures 1 or more digits into Group 1, then matches x, and then matches and captures into Group 2 one or more digits possessively without letting backtracking into the digit matching pattern, and the (?!-) negative lookahead check (making sure there is no - immediately after the current position) is performed once after \d++ matches all the digits it can. In case of \d+(?![-\d]), the 1+ digits are matched first, and then the negative lookahead makes sure there is no digit and - immediately to the right of the current location.
• \b(\d+)x(\d+)\b(?!-) - matches a word boundary first, then matches and captures 1 or more digits into Group 1, then matches x, then matches and captures into Group 2 one or more digits, then asserts that there is a word boundary, and only then makes sure there is no - right after.

See a PHP demo:

if (preg_match('~(\d+)x(\d++)(?!-)~', "18x18", $m)) {
    echo "18x18: " . $m[1] . " - " . $m[2] . "\n";
}
if (preg_match('~\b(\d+)x(\d+)\b(?!-)~', "18x18", $m)) {
    echo "18x18: " . $m[1] . " - " . $m[2] . "\n";
}
if (preg_match('~(\d+)x(\d++)(?!-)~', "18x18-", $m)) {
    echo "18x18-: " . $m[1] . " - " . $m[2] . "\n";
}
if (preg_match('~\b(\d+)x(\d+)\b(?!-)~', "18x18-", $m)) {
    echo "18x18-: " . $m[1] . " - " . $m[2];
}

Output:

18x18: 18 - 18
18x18: 18 - 18