Regex to match strings in between similar delimiters
===

I have a long  string

Blah Blah Blah
*****Some text 1 blah*****
More Blah...
*****Some text 2 blah*****
blah bleh...

I want to extract the strings in between "**" so I would get
>    Some text 1 blah

>    Some text 2 blah

I tried using:

    var expression = @"(?<=**)(.*?)(?=**)";
    var matches = Regex.Matches(theLargeString, expression);
but it is throwing an error:

    parsing "(?<=**)(.*?)(?=**)" - Quantifier {x,y} following nothing.

I am new to Regex... Any advices?