It appears that your strings are UTF-8; not extend ASCII. Furthermore, printing your strings through od shows that the byte values that you said you wanted to remove are not present in your input string or output string samples:

Code:

printf '%s' 'testing_Š_testing' | od -t cu1
printf '%s' 'testing__testing' | od -t cu1

shows us that the unsigned decimal byte values of the two bytes you want to remove are 197 and 160:

Code:

0000000    t   e   s   t   i   n   g   _   Š  **   _   t   e   s   t   i
          116 101 115 116 105 110 103  95 197 160  95 116 101 115 116 105
0000020    n   g                                                        
          110 103                                                        
0000022
printf '%s' 'testing__testing' | od -t cu1
0000000    t   e   s   t   i   n   g   _   _   t   e   s   t   i   n   g
          116 101 115 116 105 110 103  95  95 116 101 115 116 105 110 103
0000020

If you are working with UTF-8 input and want "extended ASCII" output (where you may be removing 1 or more bytes out of a multi-byte UTF-8 character, but might not be removing complete characters), you may end up with an unintelligible mess. If you want to remove a specific set of UTF-8 characters, that is easy to do. If you want to remove all non-(7-bit)ASCII characters, that is easy to do on some systems (depending on how well your version of awk handles locales and multi-byte characters).

What OS (including version) and shell are you using?

What Locale are you using when your run this script?

Is it OK to just remove all bytes from your input stream that have the high order bit set? If not, is there a specific list of UTF-8 characters you want to remove? If not, and you really want to remove individual bytes from strings containing multi-byte characters, this may be hard to do in some versions of awk.

You said you know how to do what you want using sed. Show us the sed substitute command that does what you want and we can show you how to easily change that into an awk sub() or gsub() function call.