Sed regular expressions

[Jimmy B]

Hey there,

Im trying to search and replace some text in a file using sed with some regular expressions. Basically I have:

BGQ032L24
BGQ032N02
BGQ032N04
BGQ032N06
BGQ032N08
BGQ032N10
BGQ032N12
BGQ032N14

And I want to insert a character (underscore) between before the third last character like this:

BGQ032_L24

I tried:
sed 's/BGQ[0-9][0-9][0-9]/BGQ[0-9][0-9][0-9]/g' file
and I get the search fine but the regular expression adds to the replacement.

Any suggestions?

[aquavitae]

Not entirely sure what your problem is, but don't you need to add an underscore to sed:

sed 's/BGQ[0-9][0-9][0-9]/BGQ[0-9][0-9][0-9]_/'

[Jimmy B]

Perhaps im not explaining it right (and yes i forgot the _ in my post above). If I execute the command you just suggested I get as an output:

BGQ[0-9][0-9][0-9]_
BGQ[0-9][0-9][0-9]_
BGQ[0-9][0-9][0-9]_
BGQ[0-9][0-9][0-9]_
BGQ[0-9][0-9][0-9]_
BGQ[0-9][0-9][0-9]_
BGQ[0-9][0-9][0-9]_
BGQ[0-9][0-9][0-9]_

When what I want it is:

BGQ032_L24
BGQ032_N02
BGQ032_N04
BGQ032_N06
BGQ032_N08
BGQ032_N10
BGQ032_N12
BGQ032_N14

[Some Penguin]

That's what capturing groups and back references are for.

[DaithiF]

Hi,

sed 's/BGQ[0-9]\{3\}/&_/' somefile

[DaithiF]

or alternatively, if you want to work from the end of the line, ie. always insert the _ before the 3rd last character of the line (regardless of how long the line may be), then

sed 's/.\{3\}$/_&/' testfile

[howlingmadhowie]

howlingmadhowie% echo "BGQ123N12" | sed -e 's/\(.*\)\(.\{3\}\)/\1_\2/'
BGQ123_N12


it looks nicer without the escapes:


sed -e 's/(.*)(.{3})/\1_\2/'

but it needs the escapes to work

DaithiF's second solution above is better than mine. it explicitly checks for the end of the line.

[Jimmy B]

Hi,

sed 's/BGQ[0-9]\{3\}/&_/' somefile

Brilliant thanks guys. The escape characters that were getting me