Lets break it up and remove the backslashes needed for a
Java String literal to make it easier on the eyes. Refer to the
Pattern API and
http://www.regular-expressions.info/
(?<=filename=") Non-capturing positive look-behind, matches the text
filename"
([^"]*?) Capturing group. The character class
[^"] matches any character
except " and the
*? repeats the match 0 or more times, reluctantly/lazily.
(?=")Non-capturing positive look-ahead, matches a
".
On review, the regex can be shortened by using the greedy construct.
(?<=filename=")([^"]*) also does the job.