RegEx string formatting in Notepad++

Section A: Pad to specific length

To right-pad lines with N characters using regular expressions, add N spaces to the end of the line, then group the first N characters replacing the rest.

Pass 1: Add padding characters

Find: $ Replace: ______________________________

At the end of the line add 30 spaces. (I used underscores since spaces wouldn't format on the post).

Pass 2: Trim left 30 characters

To pad a dash-delimited date at the beginning of a line, match each section accordingly.

Find: ^([[:print:]]{0,30}).*$ Replace with \1

At the beginning of the line, replace a group up to thirty printable characters followed by any remaining characters with the group.

To pick a different line-length, use n-spaces in Pass 1 then replace 30 with the length in Pass 2.

Section B: Line starting with date

Pass 1 (day of month):

Find what: ^([0-9])- Replace with: 0\1-

Replace the pattern (line starting with a single digit followed by a dash) with the padded zero, the digit, and the dash.

Pass 2 (month):

Find what: -([0-9])- Replace with: -0\1-

Replace the pattern (a single digit between two dashes) with a dash, the padded zero, the digit, and the dash.

In respond to:

12345678    TXT 19700101    0       100     20160624    100     Comment text
12345678    TXT 19700101    100     100,25  20160624    0,25    Comment text
12345678    TXT 19700101    100,25  100,5   20160624    0,25    Comment text

For 4th column:
^((?:\S+\s+){3}\d+)(\s) to \1,0\2
^((?:\S+\s+){3}\d+,\d)(\s) to \10\2

For 5th/7th column:
similar to above, just replace {3} with {4}/{6} in the rule respectively

Explanation

The 1st rule appends ,0 to numbers without ,. Now all numbers must have ,\d.

The 2nd rule appends a 0 to those with single digit after comma.

As for (?:):non-capture group, the previous columns are already captured as \1 so additional capturing is unnecessary.

This only pads number to 2 decimal places. To pad an arbitrary amount, use the pad excessively, then trim approach.

Final word?
In my opinion, plain regex as in notepad++ is inadequate for this task. Some basic scripting like bash or perl would have handled this with much higher readability.