sed replace text at based on line number dynamically - shell

Question

I am looking for bash script comments from // to /* */
I got partial working
sed -i '14s/////*/' a.c
this is working like // with */ how to add */ at the end.
Originl script
#include <stdio.h>
char buffer[10] = {'0'}; // comment1
int main()
{
printf("Hello World"); // Comment2
return 0;
}
Expected file
#include <stdio.h>
char buffer[10] = {'0'}; /* comment1 */
int main()
{
printf("Hello World"); /* Comment2 */
return 0;
}

Answer 1

Simplest solution
Assuming the idiosyncratic spacing in the desired output shown in the question is unintentional:
sed 's%// *\(.*\)%/* \1 */%'
The keys here are:
Using % instead of / to mark the separate parts of the s/// (or s%%%) command.
Capturing the text of the comment in \(…\).
Replacing it with \1 (preceded by /* and followed by */ and single spaces.
Working on a direct copy of the data from the question, the output is:
#include <stdio.h>
char buffer[10] = {'0'}; /* comment1 */
int main()
{
printf("Hello World"); /* Comment2 */
return 0;
}
Improving the handling of spaces
There are trailing blanks after the comments — ugly! We can fix that with care:
sed 's%//[[:space:]]*\(.*[^[:space:]]\)[[:space:]]*$%/* \1 */%'
That matches zero or more spaces after the // opening the comment, and matches up to the last non-space before an optional string of spaces at the end of the line. That generates:
#include <stdio.h>
char buffer[10] = {'0'}; /* comment1 */
int main()
{
printf("Hello World"); /* Comment2 */
return 0;
}
And you can deal with all trailing white space first, which is probably a good idea anyway, using:
sed -e 's/[[:space:]]\{1,\}$//' -e 's%//[[:space:]]*\(.*\)%/* \1 */%'
which yields:
#include <stdio.h>
char buffer[10] = {'0'}; /* comment1 */
int main()
{
printf("Hello World"); /* Comment2 */
return 0;
}
That differs from the previous output by not having a space after main().
Proper comment handling is hard!
Note that this simple code can easily be confused by valid C, such as:
printf("// this is not a comment\n");
To understand C fully enough not to make that mistake is beyond sensible sed. Less seriously, it will miss some valid but implausible character sequences that are officially comments, such as:
/\
/this is a comment\
and this is also part of the comment\
even with extra spaces
and if you allow trigraphs (don't), then:
/??/
/??/
This is part of the comment started two lines before!
This sort of stuff shouldn't afflict any actual code base, but are the sorts of garbage that compiler writers have to handle correctly.