Sed Replace – How to Replace Pattern on Every Second Newline

replacesed

Is there a way to tell sed to replace the pattern on every second occurrence? Or at least on every second line? (Sure it's possible with a script, but I was asking myself if sed can do it to).

Edit

I found

sed -e "s/pattern/replacement/g;n"

But it replaced every first occurence, not the second.

Example

Input file:

I have a pattern in each line
Also the second line has the pattern
Another pattern here
And -- you guess it -- a pattern

Desired output:

I have a pattern in each line
Also the second line has the replacement
Another pattern here
And -- you guess it -- a replacement

Best Answer

see https://stackoverflow.com/questions/5858200/sed-replace-every-nth-occurrence

The solution uses awk rather than sed, but "use the right tool for the job". It may or may not be possible to do in sed but, even if it is, it will be a lot easier in a tool like awk or perl.

Related Solutions

sed Regular Expression – How to Substitute the First Occurrence of a Pattern in a Line for All Lines in a File with sed

You're overthinking it. sed replaces only the first instance on a line by default (without the /g modifier), although you still want to anchor because you don;t so much want the first instance in the line as the one at the start of the line; and you usually don't need the explicit line actions you're trying to use (why?).

sed 's/^" /"/'

Bash – Replace multiple strings in a single pass

OK, a general solution. The following bash function requires 2k arguments; each pair consists of a placeholder and a replacement. It's up to you to quote the strings appropriately to pass them into the function. If the number of arguments is odd, an implicit empty argument will be added, which will effectively delete occurrences of the last placeholder.

Neither placeholders nor replacements may contain NUL characters, but you may use standard C \-escapes such as \0 if you need NULs (and consequently you are required to write \\ if you want a \).

It requires the standard build tools which should be present on a posix-like system (lex and cc).

replaceholder() {
  local dir=$(mktemp -d)
  ( cd "$dir"
    { printf %s\\n "%option 8bit noyywrap nounput" "%%"
      printf '"%s" {fputs("%s", yyout);}\n' "${@//\"/\\\"}"
      printf %s\\n "%%" "int main(int argc, char** argv) { return yylex(); }"
    } | lex && cc lex.yy.c
  ) && "$dir"/a.out
  rm -fR "$dir"
}

We assume that \ is already escaped if necessary in the arguments but we need to escape double quotes, if present. That's what the second argument to the second printf does. Since the lex default action is ECHO, we don't need to worry about it.

Example run (with timings for the skeptical; it's just a cheap-o commodity laptop):

$ time echo AB | replaceholder A B B A
BA

real    0m0.128s
user    0m0.106s
sys     0m0.042s
$ time printf %s\\n AB{0000..9999} | replaceholder A B B A > /dev/null

real    0m0.118s
user    0m0.117s
sys     0m0.043s

For larger inputs it might be useful to provide an optimization flag to cc, and for current Posix compatibility, it would be better to use c99. An even more ambitious implementation might try to cache the generated executables instead of generating them each time, but they're not exactly expensive to generate.

Edit

If you have tcc, you can avoid the hassle of creating a temporary directory, and enjoy the faster compile time which will help on normal sized inputs:

treplaceholder () { 
  tcc -run <(
  {
    printf %s\\n "%option 8bit noyywrap nounput" "%%"
    printf '"%s" {fputs("%s", yyout);}\n' "${@//\"/\\\"}"
    printf %s\\n "%%" "int main(int argc, char** argv) { return yylex(); }"
  } | lex -t)
}

$ time printf %s\\n AB{0000..9999} | treplaceholder A B B A > /dev/null

real    0m0.039s
user    0m0.041s
sys     0m0.031s

Best Answer

Related Solutions

sed Regular Expression – How to Substitute the First Occurrence of a Pattern in a Line for All Lines in a File with sed

Bash – Replace multiple strings in a single pass

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