<?php $str = '۱۲۳۴۵۶۷۸۹۰'; var_export( preg_replace( '~.\K(?=(?:.{3})+$)~u', ",", $str ) ); echo "\n---\n"; var_export( preg_replace( '~([۱۲۳۴۵۶۷۸۹۰])(?=(?:[۱۲۳۴۵۶۷۸۹۰]{3})+$)~u', "$1,", $str ) );
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