$re = '`^([/#]?[A-Za-z0-9$]+)[/_].*`m';
$str = '#Tableau_6_sid:15251a_4i_2a
/15722c29d_PhotoImageCameraPro
JAVA/15722c29d_PhotoImageqeeee
ABC_CUST_INFO_D
BBC_CUST_ENTRY_F
FHS_PRDCT_STST_T
GHS_INVTR_CD_DRY';
$subst = "\1";
$result = preg_replace($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