10

I want to strip all numeric characters from a String variable, eg from:

VARIABLE=qwe123rty567

to:

echo $VARIABLE
> qwerty

I've searched many posts but they either use sed to output to file/file names, or output to echo. I was not able to get it working with because of the white space:

VARIABLE=$VARIABLE | sed 's/[0-9]*//g'

4 Answers 4

15

With bash:

$ printf '%s\n' "${VARIABLE//[[:digit:]]/}"
qwerty

[:digit:] can contain other characters than 0 to 9 depend on your locale. If you want only to remove 0 to 9, use C locale instead.

3
  • 2
    Note: depending on your locale, [:digit:] might contain more or less than you would expect it to. If you only want the numbers 0 to 9, use LC_COLLATE=C and [0-9].
    – Chris Down
    Nov 25, 2015 at 11:05
  • 1
    @ChrisDown: Yes, the question asked for numeric characters, not 0 to 9, so I used [:digit:] instead of [0-9].
    – cuonglm
    Nov 25, 2015 at 12:00
  • Yeah, it's good for the question asked. It's not uncommon for Westerners to assume that [;digit:] == [0-9] though, so worth mentioning the pitfalls :-)
    – Chris Down
    Nov 25, 2015 at 14:52
10

For variety, here's methods using

tr:

VARIABLE=$(printf '%s' "$VARIABLE" | tr -d '0123456789')

sed:

VARIABLE=$(printf '%s' "$VARIABLE" | sed 's/[0-9]//g')

Bash expansion, by far the most terse:

VARIABLE=${VARIABLE//[0-9]/}

and finally Bash expansion again, this time using the [[:digit:]] character class.

VARIABLE=${VARIABLE//[[:digit:]]/} 

Note that (as others have pointed out) [[:digit:]], should cover anything defined as a digit in your locale.

2
  • Where can I find docs for //.../? Is it a normal regex? I tried //[0-9]{2}\$/ for two numbers at the end, to no success. Nov 2, 2018 at 13:40
  • It's part of the bash variable expansion syntax, not Regexp. It means replace all matches.
    – ocodo
    Nov 2, 2018 at 23:00
3
VARIABLE=qwe123rty567
IFS=0123456789
set -f # Disable glob
printf %s $VARIABLE

qwerty

further manipulation is possible.

VARIABLE=qwe123rty567
IFS=0123456789
set -f # Disable glob
set -- $VARIABLE
IFS=;   VARIABLE=$*
printf "replaced $# numbers in \$VARIABLE. RESULT:\t%s\n" "$*"

replaced 6 numbers in $VARIABLE. RESULT:    qwerty
2
  • hmm, my variable becomes "qwe rty" with 3 spaces in the middle and 3 at the end
    – George
    Nov 25, 2015 at 10:48
  • @George - you're probably using echo which replaces nulls with spaces - like eval.
    – mikeserv
    Nov 25, 2015 at 10:49
2

White space (actually, failure to quote your variables) was only part of the problem.

You can't just pipe a variable through sed like that, it doesn't work. More precisely, it doesn't pipe the value of "$VARIABLE" through sed, the shell will try to execute the value of "$VARIABLE" and pipe the output of that through sed. BTW, this is not a bug - this is useful if $VARIABLE happens to contain a valid command like ls or rsync or whatever.

Also, if you want to assign the ouput of a command or pipeline to a variable, you need to surround that command/pipeline with $().

So, to modify a variable with sed, you need to do something like this:

VARIABLE=$(printf '%s' "$VARIABLE" | sed 's/[0-9]*//g')

You could use echo there instead of printf but echo will interpret and act on certain character sequences in $VARIABLE (e.g. \t, \n, \r, etc), while printf won't. You'll run across a lot of examples using echo...replace them with printf '%s', it's much safer.

4
  • Is there any real reason to single quote the %s ?
    – Wildcard
    Nov 25, 2015 at 22:53
  • 1
    @wildcard. %s by itself, no. more complicated format strings should be quoted, though, so it's a good habit to get into.
    – cas
    Nov 25, 2015 at 22:55
  • there was no failure to quote any variables - everything was quoted just fine. and white-space isn't even involved (and rarely actually is). i know the asker mentions it as a possible source of whatever the issue was, but it probably isn't helpful to encourage wrongful assumptions. your command substitution will strip white-space from the tail of whatever is in $VARIABLE, though.
    – mikeserv
    Nov 26, 2015 at 3:25
  • and you're making that judgement on one made up example of VARIABLE=qwe123rty567? OK, right. I made my assessment ("failure to quote your variables") on the basis of none of the OP's examples having quotes around the variables, and the fact that he mentioned white-space as being a problem.
    – cas
    Nov 26, 2015 at 4:34

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