$re = '~\b[A-Za-z2]+(?=\()(?![^\']*\'!)~';
$str = '=VLOOKUP($A9,\'Summary\'!$A$10:$C$30,3,FALSE)
=IFERROR((C10/B10),"N/A")
=\'New Chart Data (Date)\'!L70
=IFERROR((C10/B10),Len(E30))
=\'New Chart Data(Date)\'!L70 + (Len(5)&ROW())
=INDEX(C$3:C$12,MATCH(9^99,IF(MAX(IF(ABS($B$3:$B$12-$B18)=MIN(IF($A$3:$A$12=$A18,ABS($B$3:$B$12-$B18),999)),B$3:B$12))=IF(ABS($B$3:$B$12-$B18)=MIN(IF($A$3:$A$12=$A18,ABS($B$3:$B$12-$B18),999)),B$3:B$12),IF(ABS($B$3:$B$12-$B18)=MIN(IF($A$3:$A$12=$A18,ABS($B$3:$B$12-$B18),999)),C$3:C$12))))';
$subst = "6";
$result = preg_replace($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