If you don't mind my whoring my own tutorial (you may have read it), the idiom you're looking for is:

Matching a pattern that doesn't include another pattern

You might want to capture everything between foo and bar that doesn't include baz. The technique is to have the regex engine look-ahead at every character to ensure that it isn't the beginning of the undesired pattern:

/foo  # Match starting at foo
 (         # Capture
 (?:       # Complex expression:
   (?!baz) #   make sure we're not at the beginning of baz 
   .       #   accept any character
 )*        # any number of times
 )         # End capture
 bar  # and ending at bar
/x;
[download]

Note that you have to have the lookahead checked for every character you're accepting. In your case, the [^\n]+ sub-pattern needs to be adjusted to make sure every character is not the beginning of a Lib:

 (?:       # Complex expression:
   (?!Lib) #   make sure we're not at the beginning of Lib
   [^\n]       #   accept any character
 )+        # any positive number of times
[download]