How to match )\n{ using sed?

Question

I need to write a shell script which will input a string in my .c files after a quite complicated pattern to match.

The pattern is: )\n{,

without any tabs/spaces between the \n and the {. That's to say that I want to match with a { located in the first column of my file (this in order to ONLY match with )\n{ following function declarations in C, not the ones following loops or conditions).

void some_function_declaration(char var1, char var2) {

I have read manuals, forums and still cannot figure the right way to write the pattern to match or find the corresponding regex. The corresponding output would be:

void some_function_declaration(char var1, char var2) { time_exe(__func__, cl(clock())); rest of the function...

What follows is what I've came up with so far and doesn't work.

Try 1

sed -i '' '/)$\n{/a time_exe(__func__, cl(clock()));' >> list_func2.c

Try 2

sed -i '' '/)\n{ /a time_exe(__func__, cl(clock()));' >> list_func2.c

Try 3

sed -i '' '/)/\n{/a time_exe(__func__, cl(clock()));' >> list_func2.c

I would be very glad to hear your recommandations about this matter.

Answer 1

You should avoid using sed which is a line-based tool and won't handle this kind of task well.

If you insist on using sed and are using GNU sed, you can however use the -z / --null-data option which will read the whole file in one pass (reading NUL-byte separated records rather than linefeed-separated records) and will enable you to use the )\n{ pattern as you would expect :

$ { echo "line1)"; echo "{line2"; } | sed -z 's/)\n{/X/g'
line1Xline2

As this requires loading the whole file in memory, expect terrible performances for huge files.

---

If you like unmaintainable gibberish you can solve this problem using sed's less known P, t and D commands :

sed '/)$/{N;s/)\n{/) {\n\ttime_exe(__func__, cl(clock()));/;t;P;D}'

Try it here !

This works by loading an additional line in the pattern space (N) when a line ending in ) is encountered, trying to substitute against the two-lines pattern, and printing (P) and removing (D) the first line from the pattern space if the pattern isn't matched (otherwise t branches to the next iteration), leaving the second line in the pattern space to be used as the first line of the next iteration.

Using /first line pattern/{N;s/whole pattern/replacement/} is often good enough, but it can fail as N will consume a line you won't test the first line pattern against. This is illustrated here.

Answer 2

I agree with @Aaron, but if you still want exactly sed, look at this:

$  cat /tmp/del.txt
void some_function_declaration(char var1, char var2)
{
  ...
}
void enother_function_declaration(char var1, char var2)
{
  ...
}

And applying sed:

$ cat /tmp/del.txt | sed ':a;N;$!ba;s/)\n{/)\{\ntime_exe(__func__, cl(clock()));/g'
void some_function_declaration(char var1, char var2){
time_exe(__func__, cl(clock()));
  ...
}
void enother_function_declaration(char var1, char var2){
time_exe(__func__, cl(clock()));
  ...
}

I think it looks like you wanted

UPD

Let me explane..

More cross-platform compatible syntax is:

sed -e ':a' -e 'N' -e'$!ba' -e 's/)\n{/)\n{ ... ;/g'

where

• ':a' - creates a branch label (named a) that can be returned later
• 'N' - appends next line to current (with \n between)
• '$!ba' - jumps to label a if next line is the last line
• 's/)\n{/)\n{ ... ;/g' - makes global substitution in the single line, composed of all the lines and \ns