\+ with sed doesn't work

Question

I'm using Bash and \\+ works with grep but not with sed.

For example if I do

echo "abbbbc"| sed -e s/ab\\+c/def/

I obtain this result : abbbbc

I tried with ' or with " around s/ab\\+c/def/, I obtain the same result.

but if I replace \\+ with *, I obtain : def

I have to change >Atab_TR4682|c0_g1_i1|m.14206 into Atab

If I do echo ">Atab_TR4682|c0_g1_i1|m.14206" | sed -e "s/>*\\([[:alpha:]]\\\*)_.\*/\1/g"

I obtain Atab_TR4682|c0_g1 probably because * could also be used for 0 iteration, but if I replace * by \\+ it doesn't work at all..

Does anyone have an explanation?

unable to reproduce iow works fine here - what's your sed version ?; @DopeGhoti - with ERE he'll have to remove the \ before the + — don_crissti, Mar 24 '17 at 17:35
Is your sed already aliased to sed -r (or sed -E) maybe? That would turn \+ into + and vice versa — steeldriver, Mar 24 '17 at 17:46
http://unix.stackexchange.com/questions/119905/why-does-my-regular-expression-work-in-x-but-not-in-y — ilkkachu, Mar 24 '17 at 17:55
I checked man sed and there was no -E switch, but there was -r, --regexp-extended. What I meant about the quotes, @don_crissti, is that if not in strong quotes, bash will catch the escape before it'll even get to sed. — DopeGhoti, Mar 24 '17 at 17:56
@DopeGhoti - yes, without quotes and with a single backslash it will work but as you can see OP is using double backslash... — don_crissti, Mar 24 '17 at 18:02

Fox · Answer 1 · 2017-03-24T17:48:12.473

1

GNU sed accepts an escaped +, so if your sed is GNU-compatible, you can do:

$ echo "abbbbc" | sed 's/ab\+c/def'
def
$ echo "abbbbc" | sed "s/ab\+c/def"
def
$ echo "abbbbc" | sed s/ab\\+c/def
def

The POSIX-specified (more generally available) sed command only uses POSIX BREs (basic regular expressions) by default. If your sed is POSIX-compatible but not GNU-compatible and you want x+ to act as xx*, then you want the -E switch:

$ echo "abbbbc" | sed -E 's/ab+c/def/'
def

Some non-GNU implementations of sed include -r as a synonym for -E, for compatibility with older versions of GNU sed. The -E syntax is POSIX, and recent GNU sed accept both.

edited Mar 24 '17 at 17:48

answered Mar 24 '17 at 17:35

Fox

8,193

@don_crissti Edited – Fox Mar 24 '17 at 17:48
Yes but isn't that (sed s/ab\\+c/def) what the OP claims that it's not working as expected ? – don_crissti Mar 24 '17 at 17:50
@don_crissti Yes — my assumption is that their sed is non-GNU (mine certainly is) – Fox Mar 24 '17 at 17:51
Let's see if the OP bothers replying to the comments... Anyway, if their grep supports \+ that means it's a gnu setup hence gnu sed – don_crissti Mar 24 '17 at 17:55

\+ with sed doesn't work

1 Answers1