Description:
------------
Fails to compile pattern.
Workaround:
put laziness switch  `?` right after `+` quantifier:
...
 +?           # 1 or more times as short as possible(lazy matching)  
...

Reproduce code:
---------------
//fetch image URLs
$pattern = '~
<           # open tag
  img         # name
  \s          # whitespace 
  +           # 1 or more times
  src=        # image source attribute equals
  "         # open quote
  (           # begin group capture 
    [^"]        # URL: everything up to quote
    +           # 1 or more times
    ?           # as short as possible(lazy matching)    
  )           # end group capture
  "         # close quote 
  [^>]        # any except >
  *           # 0 or more times 
>           # close tag
~xi'; //omit whitespaces, case insensitive
preg_match_all($pattern, '<img src="test"/>', $matches);
echo $matches[1];

Expected result:
----------------
test

Actual result:
--------------
PHP Warning:  preg_match_all(): Compilation failed: nothing to repeat at offset [...stripped]