Ubuntu – How to replace strings in file without regex

Unfortunately, it appears you cannot; and that it's an incomplete regex implementation at fault.

This line at the end of all the GPRename locale translations seems to make that obvious:

msgid "Backreferences with Regular expression cannot be use
       as $1 is considered a Perl variable."

Since you are familiar with Perl regular expressions, may I recommend you use the excellent rename command-line utility instead?

• Always use the -n switch first to get a preview.
• There's a typo in your regex - it should be \((\d{2})\), with the grouping inside the escaped parentheses.

• Example:

  $ rename -n 's/\((\d{2})\)/$1/' *.avi
  (01) title of file1.avi renamed as 01 title of file1.avi
  (02) title of file2.avi renamed as 02 title of file2.avi
  (03) title of file3.avi renamed as 03 title of file3.avi
  

• Call rename with the -v (verbose) switch and drop the -n to actually perform the renaming.

To delete any line that is not composed entirely of alphabetic characters, you'd need to add start (^) and end ($) anchors

sed '/^[[:alpha:]]*$/!d' file

Instead, you could delete any line that contains at least one non-alphabetic character

sed '/[^[:alpha:]]/d' file

Note that the caret ^ is acting as a negation operator here rather than as an anchor as in the previous expression.

Alternatively, using grep's whole-line (-x or --line-regexp) option

grep -x '[[:alpha:]]*' file

(equivalent of the first sed expression) or using an inverse match (-v)

grep -v '[^[:alpha:]]' file

(equivalent of the second sed expression).