Regular Expression for formatting a file

The challenge here, as Mohan implied, is finding the "negative" match. It's straightforward to find a line that starts with 10 digits and a pipe, but how do you find one that doesn't?

Here's a regex that will do it:

^([^|]*[^|\d][^|]*\||.{10}[^|])

It matches only the last 4 of these input lines:

1000000179|abcd.....|abc|
1000000180|wedwedw...|234|
1000000179|abcd.....
%d20000180|wedwedw...
3214a23642|abcd
123456789|whatever
1234567890_abcde

Breaking down the regex, it's looking for one of two conditions at the beginning of a line:
1) Any non-digit before the first pipe ( [^|]*[^|\d][^|]*\| )
2) Any non-pipe character in the 11th position ( .{10}[^|] )

The pattern to check for a valid line would be :

string pattern = @"^\d{10}\|.*$";

Which would match if the line is beginning with 10 digits, followed by a pipe, followed by any number of any character.

Just test for non-numeric characters in the first 10.

See regex for VALID line:

^\d{10}\|

It's easy to define the valid lines, but slightly more difficult to define the invalid lines.
So, you define the (in pseudo code) line = valid or invalid.
Even it sounds trivial, this covers all the lines.

Loop over all lines and process the matches where the invalid part is available.
E.g.

string filePath = @"...";
string data = File.ReadAllText(filePath);
string linePattern = @"^(?:\d{10}\|.*|(.*))$";
var invalidLines = from m in Regex.Matches(data, linePattern, RegexOptions.Multiline).Cast<Match>()
                   where m.Groups[1].Success
                   select m.Groups[1].Value;
foreach(string invalidLine in invalidLines)
{
   //...
}

Since the above regex pattern is greedy, it takes the first match, which is either the valid one (\d{10}\|.*) or the invalid one (.*). The two are separated by the or operator (|).
To get access to the invalid data, it is enclosed in parenthesis ((.*)).
To limit the pattern to a line each, the whole pattern is enclosed in ^...$ and grouped by a non-referencing group ((?:...), i.e. a group that does not count in the Groups array of a match.

Putting it all together results in ^(?:\d{10}\|.*|(.*))$.
Note that the match option is set to Multiline to give the ^ and $ the needed meening: begin/end of each line (where in Singleline mode the ^ and $ would mean begin/end of the whole string).

Cheers
Andi