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This should be straight-forward, but I cannot figure it out. If I want to replace an A or B with a C using sed the code could potentially be:

$ echo AAXXAAYYBB | sed 's/[AB]/C/g'
CCXXCCYYCC

This results in all A's and B's converted to C's.

What I'd like to do is replace "A" with either one of two (or potentially more) variables:

Input:

AAXXAAYYBB

Code:

sed 's/A/[BC]/g'

Output (where the substitution of B or C is random):

BCXXCBYYBB 

But this code will only change A's to...

$ echo AAXXAAYYBB | sed 's/A/[BC]/g'
[BC][BC]XX[BC][BC]YYBB

I'm trying to avoid PERL here if possible. Does anyone have an idea how to fix this?

Plenty of ways to skin this particular cat once we resort to scripting, but here is something I threw together - It may not be pretty, (and relies on a bash shell!) but it might help you out:

#!/bin/bash

TEXT="AAXXAAYYBB"

echo "Start: $TEXT"

# So long as there are un-converted 'A' in the input string...
while [[ "$TEXT" =~ A ]]
do
 # .. convert one 'A' to a random choice of either 'B' or 'C'
 TEXT=$(echo $TEXT | sed -e "s/A/$(((RANDOM%2>0))&&echo B || echo C)/")

 # lets show how we are progressing...
 echo "Progress: $TEXT"
done

# No more 'A' in the input string, we are done:
echo "End: $TEXT"

Example output:

First run:

Start: AAXXAAYYBB

Progress: BAXXAAYYBB

Progress: BBXXAAYYBB

Progress: BBXXBAYYBB

Progress: BBXXBCYYBB

End: BBXXBCYYBB

Second run:

Start: AAXXAAYYBB

Progress: CAXXAAYYBB

Progress: CBXXAAYYBB

Progress: CBXXCAYYBB

Progress: CBXXCBYYBB

End: CBXXCBYYBB

It is possible to replace the first match of an string with:

$str/A/...

And, it is possible to generate a random (not a cryptographic secure number) value with:

r=(B C)
$r[RANDOM%2]

Each time the variable r is expanded.

So:

#!/bin/bash

str=AAXXAAYYBB
r=(B C)

while [[ $str =~ A ]]; do
 str=$str/A/"$r[RANDOM%2]"
done

echo "str=$str"

Will generate a random result each time it is called.

It is much more "straightforward" to replace A or B with C than to replace A with B or C, as the or in the former case has nothing to do with randomness, while in the latter it's based on it. So while the former could be broken down into two simple steps:

1. replace A with C


2. replace B with C

In the latter case there's the whole lot of deciding which one should be the replacement in every particular case, B or C? What's the basis? How random should the randomness be?

sed doesn't offer any random operations as far as I know. (While Perl should be a good tool.)

See related:

• How to generate a random string?




• And other random tagged questions

Not Sed, but avoids Perl:

$ echo AAXXAAYYBB | gawk '
 BEGINsrand() 
 
 n = patsplit($0,a,/A/,s); 
 for(i=1;i<=n;i++) printf("%s%s", rand() < 0.5 ? "B" : "C", s[i]); 
 print ""
 
 '
CBXXCCYYBB