Great!

Hmmmm... it's hard to say without an example. It is possible that the import from the text file is creating a data structure that is somewhat different from what you are expecting. Does the issue happen with all 3 approaches that I suggested? And, if so, does it happen in the same way? How large is the file that shows the problem? If it is email-able (less than 5 meg) then email it to the address that is posted on the "contact" section of my website (found from clicking on my name here).

By the way, the reason why you may have thought that the three dashes were not strings is that string characters (the quotation marks) are by default suppressed in output cells. So if you execute this in an input cell

"I am a string"

The output cell will look like this

I am a string

Is the list that you obtain when it is imported into Mathematica this,

{{1,2,3,---},{4,5,---},{---,---,---},{7,8,9,10,---}}

or is it this:

{{1,2,3,"---"},{4,5,"---"},{"---","---","---"},{7,8,9,10,"---"}}

I.e., are the dashes imported as a string of dashes? Without them being strings, the expressions (of 3 dashes in a row) are not syntactically valid. So I will assume that they appear as strings. If so then the following will work for you:

In[1]:= test = {{1, 2, 3, "---"}, {4, 5, "---"}, {"---", "---", "---"}, {7, 8, 9, 10, "---"}}

In[2]:= DeleteCases[test /. "---" -> Sequence[], {}, Infinity]

Out[2]= {{1, 2, 3}, {4, 5}, {7, 8, 9, 10}}

Another way to do it might be

In[3]:= DeleteCases[DeleteCases[test, "---", Infinity], {}, Infinity]

Out[3]= {{1, 2, 3}, {4, 5}, {7, 8, 9, 10}}

And here's another....

In[4]:= (test /. "---" -> Sequence[]) /. {} -> Sequence[]

Out[4]= {{1, 2, 3}, {4, 5}, {7, 8, 9, 10}}