UNIX for Dummies Questions & Answers

How to replace a character followed by a digit using sed? For example lets say I have this file -

Code:

a1       3242134    54235435   3241235
a2       3214345    45325626   3125435
a3       4236577    54365376   6865678
.
.
.
a3000  5432534    32546546   3254365

I want to replace all a1,a2,a3..a3000 with aw. I tried thi:s sed -i 's/^aa*[0-9]/aw/g' file_name. The problem is it replaces a1..a10 but then it starts counting aw0,aw1...aw9, then again aw0..aw9 upto a100. Then it starts aw0,aw1..aw999. How do just I erase these numbers? Thanks a lot!

Do this print what you want?

Code:

sed -e 's/^a[0-9]\+/aw/'

Yes!! Thanks man! You're a lifesaver! I almost gave up and was trying to do that using Matlab. Thanks a lot.

This will work with some versions of sed, but it isn't portable.

The following should work with any version of sed:

Code:

sed -e 's/^a[0-9][0-9]*/aw/'

Little clue gave it away

The standards say that sed uses Basic Regular Expresions. In BREs [0-9]\+ matches a single digit followed by a literal plus sign. Some versions of sed ignore the standard on this and treat [0-9]\+ as an RE that will match one or more digits. The way to match one or more digits portably using standard BRE syntax is [0-9][0-9]* (which matches a single digit followed by zero or more digits). And, you're right in noting that we both missed the -i option the OP used. Looking at what was requested by the OP again, it might have been better to suggest:

Code:

sed -i 's/^a[1-9][0-9]*/aw/'

since it doesn't look like a leading zero should be allowed in the string of digits following the a or A at the start of the line.

Note also that ^a[123]\([0-9]\{1,3\}\)\?\+ (on systems that accept that form of RE) wouldn't do what was wanted even if it was a valid BRE): it precludes a4 through a9, a40 through a99, and a400 through a999 which, presumably, would have been wanted as part of the vertical elipsis between the lines starting with a3 and a3000.

If the OP wanted a leading a or A followed by any string of alphanumerics ending with a digit, that would be:

Code:

sed -i 's/^a[[:alnum:]]*[0-9]/aw/'

or:

Code:

sed -i 's/^a[[:alnum:]]*[0-9] /aw /'

And, of course, both of us are still assuming that the g at the end of the sed substitute command should be ignored and that the anchor at the start of the RE was intended to take precedence over the g flag at the end.

Sorry, I edited my post because, I thought it was irrelevant what I was going to propose.

I would have been wrong as well because I believe it would have done a3000 - a3999