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@ 2018-02-18T11:55:02Z

<?php

$strings = [
    "(a+b)",
    "(a+b)(ab+ba)",
    "(a+b)(ab+ba)(ab+ba)",
    "(ab+ba)",
    "a+b",
    " (a+b)(, (a+b)a"
];

$pattern = '/^\(a\+b\)(?:\(ab\+ba\))*$/';
foreach ($strings as $string) {
    preg_match($pattern, $string, $matches);
    print_r($matches);
}

Output
Performance
VLD
References
Branches

preg_match: documentation ( source)
print_r: documentation ( source)

preferences:dark modelive previewkey bindings

31.71 ms | 405 KiB | 5 Q