For example

 (?<=This is)(.*)(?=sentence)

[Regexr][1]

I used lookbehind `(?<=)` and look ahead `(?=)` so that "This is" and "sentence" is not included in the match, but this is up to your use case, you can also simply write `This is(.*)sentence`.

The important thing here is that you activate the "dotall" mode of your regex engine, so that the `.` is matching the newline. But how you do this depends on your regex engine.

The next thing is if you use `.*` or `.*?`. The first one is greedy and will match till the last "sentence" in your string, the second one is lazy and will match till the next "sentence" in your string.

**Update**

[Regexr][2]

 This is(?s)(.*)sentence

Where the (?s) turns on the dotall modifier, making the `.` matching the newline characters.

**Update 2:**

 (?<=is \()(.*?)(?=\s*\))

is matching your example "This is (a simple) sentence". See here on [Regexr][3]


 [1]: http://regexr.com?2tr28
 [2]: http://gskinner.com/RegExr/?2tr2n
 [3]: http://regexr.com?2tr5t