$re = '/^(?=\# ?\d+\.)/m';
$str = '# This is a comment <-beacuse it starts at the begining of the file
# This is a comment <- see above
# 1. Item one <- not a comment because it starts with 1.
# Description of Item 1 <- not a comment as it is after a line that starts with a number
data point 1
data point 2
data point etc
3 <-- represents number of data points under Item one
# 2. Item two <-- not a comment
# Description of item 2 <-- not a comment
data point 1
data point ..
data point 100
100
#3. Item three <--- not a comment
# Item three description
0';
preg_match_all($re, $str, $matches, PREG_SET_ORDER, 0);
// Print the entire match result
var_dump($matches);
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