替换最后出现的 HEX 文件 - Perl 正则表达式

Question

I'm building a script in bash that edits a binary file. I managed how to achieve first occurrence replace and all occurrences replace but i cannot manage how to replace the last occurrence.

The script has to replace F8 byte with F0.

This is the code for ALL and FIRST:

ALL

perl -pe 's/\x{F8}/\x{F0}/g' < CS.midi > CSnew.midi 

FIRST

perl -pe 's/\x{F8}/\x{F0}/' < CS.midi > CSnew.midi 

Any suggestions for replace the last occurrence of F8?

Answer 1

Using regular expressions isn't even needed:

perl -0777 -pe 'substr $_, rindex($_, "\xF8"), 1, "\xF0"' CS.midi > CSnew.midi

reads the entire file, and replaces the last occurrence of the matching byte.

Answer 2

First of all, your code to change the first is incorrect. You are reading the file a line at a time, and it changes the first of each line . This means is changes the first, and the first after each LF (0x0A).

-0777 will cause the entire file to be treated as one line.

perl -0777pe's/\xF8/\xF0/g'        # All
perl -0777pe's/\xF8/\xF0/'         # First
perl -0777pe's/^.*\K\xF8/\xF0/s'   # Last