$re = '/(?:(:61:[0-9]{1,6}[0-9]{4}C[0-9]+,[0-9]{0,2})(NMSC.+)(\r?\n:86:RGT FACT)|\G(?!^))[ .](\d{5})/m';
$str = ':61:2204210421C1339,57NMSCTOPF2510474511//GBBK031SCT TOPF2510474511
:86:RGT FACT 17133 TANQ BROERS SA/RGT39370 TANQ BROERS SA48 AVENUE D\'ABCDE
:61:2204270427C4808,37NMSCTOPF2520477320//GBJ6009SCT TOPF2520477320
:86:RGT FACT 17274.17442.17546 TANQ BROERS SA/RGT39370 TANQ BROERS SA48 AVENUE D\'ABCDE
:61:2203290329C5518,16NMSCTOPF2485471711//GBCJ001SCT TOPF2485471711
:86:RGT FACT.16794 16918 17079 TANQ BROERS SA/RGT39370 TANQ BROERS SA48 AVENUE D\'ABCDE';
$subst = "$1$2$3INV$4";
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php