How to replace a question mark in sed

With Sparrow/Raku it is quite simple:

task.bash

#!bash
echo "a?c"
echo "a?d"
echo "a?f"

task.check

# find all strings with "a?"
a?

code: <<HERE
!raku

for matched()<> -> $l {
  say $l.subst("a?","b?",:g)
}
HERE

Example output

s6 --task-run .
22:48:34 :: [sparrowtask] - run sparrow task .
22:48:34 :: [sparrowtask] - run [.], thing: .
[task run: task.bash - .]
[task stdout]
22:48:35 :: a?c
22:48:35 :: a?d
22:48:35 :: a?f
[task check]
stdout match <a?> True
# b?c
# b?d
# b?f

Regular Expressions are so difficult to write that you shouldnt have to write a slightly different one for every language. The partial solution to this problem is to use PCRE or Perl Compatible Regular Expressions.

Grep supports PCRE with the -P option as well as most major languages. However sed does not, even though there was once apparently something in the works. In my opinion Perl is superior to grep sed and awk in nearly every way. So the best option would probably be to just use Perl. You would not have to learn all the different wrinkles in syntax, and your regular expressions would be portable across any language that supports PCRE.

For this and many other reasons, Perl is a perfect drop in replacement for comparable languages and tools, and your original regular expression would have worked unaltered. Try something like this...

$ echo "a?c" | perl -pe 's/a\?/b\?/'
b?c

The -p option tells Perl to loop over each line of input, and print the result at the end of each line. In case you wanted to do something like this...

$ ls | perl -pe 's/a\?/b\?/'

The -e option says the entire program is on the command line instead of in a file. If you pipe input into Perl using the | operator, Perl will loop over the text of each line. But if you use regular command line arguments, Perl will assume they are files, open each file, and process them line by line. Similar to running grep with command line arguments.

With a little time and effort, you will find Perl invaluable. The syntax is very stable, supported on every platform, and Perl is just all around much more powerful and versatile than comparable tools.