How to collect elements in one list with the constraints provided in a second list

Question

I wish to collect the items in the first list provided the constraints in the second list. For example in the lists shown below the first list shows a bunch of strings and the second list shows the positions of the strings as they appear in a string:

{StringCases[#1, RegularExpression[#2]],
StringPosition[#1,RegularExpression[#2]]} &["tthis is a book fine", "(?:is|book|t)"]

{{"t", "t", "is", "is", "book"}, {{1, 1}, {2, 2}, {4, 5}, {7, 8}, {12, 15}}}

The output I am desperately trying to get should look like as follow:

{"tt","is","is","book"}

The rational behind the result is that since the first two elements are "t" and differ only by one unit which is the distance between them so they occur consecutively i.e. 1+1 = 2.

On the other hand however, the first instance of "is" occupies position 4-5 in the string and the second occurence of "is" is situated at positions 7-8, and is separated by a space (they do not occur consecutively as 4+2 = 6 and not 7). Therefore "is" do not merge.

I am struggling to find a way and would very much appreciate if someone can kindly help me with the problem. Thanks in advance !

Answer 1

As discussed in comments, we can get the result directly.

StringCases["tthis is a book fine", Repeated /@ {"is", "book", "t"}]

{"tt","is","is","book"}