sed相当于PCRE的`。？`

Question

I'm trying to find all instances of aaa and replace it with bbb but preserving the first character preceding the aaa. Here's how I'd do it in PHP with PCRE:

preg_replace('#(.?)aaa#', '\1bbb', 'aaasdfg');

How would I do something like that with sed? Here's my attempt (didn't work):

sed -i.bak -r 's/(.\?)aaa/\1bbb/g' filename.ext

It's a bit of a contrived example. What I'm trying to do is a little more complicated but, long story short, I'm trying to get .? working.

Any ideas?

Answer 1

Just now I mis-read your question, I thought you want to do .*? with sed. ..

Ok, now I understand what you mean. In another question from you, I mentioned, for BRE, you have to escape those chars to give them special meaning. But for ERE, you have to escape chars which have special meaning to get literal string.

You used -r, to let sed use ERE, but you escaped ?, it means, you want to match literal string ?.

try this:

sed -i.bak -r 's/(.?)aaa/\1bbb/g' filename.ext

or this:

sed -i.bak 's/\(.\?\)aaa/\1bbb/g' filename.ext

test:

default with BRE

kent$  echo "aaasdf
xaaasdf"|sed 's/\(.\?\)aaa/\1bbb/'
bbbsdf
xbbbsd

with -r, ERE:

kent$  echo "aaasdf
xaaasdf"|sed -r 's/(.?)aaa/\1bbb/' 
bbbsdf
xbbbsdf