$re = '/(Every )(month|\d months)(.*)/m';
$str = '<select class="wcsatt-options-product-dropdown" name="convert_to_sub_dropdown2495">
<option selected="true" value="1_month">Every month for $11.25 (10% off)
</option><option value="2_month">Every 2 months for $11.25 (10% off)
</option><option value="3_month">Every 3 months for $11.25 (10% off)
</option><option value="4_month">Every 4 months for $11.25 (10% off)
</option><option value="5_month">Every 5 months for $11.25 (10% off)
</option><option value="6_month">Every 6 months for $11.25 (10% off)
</option>
</select>';
$subst = "$1$2";
$result = preg_replace($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