$re = '/^(?:[a-z]\)|\(\d+\))[^#]*?(?=\n\(\d\)|\Z|#|\n[a-z]\))/sm';
$str = '#1
(1) This is a test.
a) This is a subtest one.
b) And another one.
(2) A really cool test.
(3) Here is the problem, text for each numbered line is
supposed to be on a single line like in (1) and (2), but
the text often spans multiple lines of text.
(4) How can I match the multi-line entries and unwrap them to single lines?
#2
(1) This is a test.
a) This is a subtest one.
b) And another one.
(2) A really cool test.
(3) Here is the problem, text for each numbered line is
supposed to be on a single line like in (1) and (2), but
the text often spans multiple lines of text.
(4) How can I match the multi-line entries and unwrap them to single lines?
#3
(1) This is a test.
a) This is a subtest one.
b) And another one.
(2) A really cool test.
(3) Here is the problem, text for each numbered line is
supposed to be on a single line like in (1) and (2), but
the text often spans multiple lines of text.
(4) How can I match the multi-line entries and unwrap them to single lines?';
preg_match_all($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