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<?php var_export( preg_replace('~(?|\d{2}(?=\d{9}$)|\d{3}(?!$))\K~', ' ', '90000800123987') );
Output for 7.3.0 - 7.3.33, 7.4.0 - 7.4.33, 8.0.0 - 8.0.30, 8.1.0 - 8.1.33, 8.2.0 - 8.2.29, 8.3.0 - 8.3.25, 8.4.1 - 8.4.12
'900 00 800 123 987'
preferences:
131.47 ms | 407 KiB | 5 Q