Only get alphanumeric characters in capture group using sed - regex

Question

Input:
x.y={aaa b .c}
Note that the the content within {} are only an example, in reality it could be any value.
Problem: I would like to keep only the alphanumeric characters within the {}.
So it would be come:
x.y={aaabbc}
Trial 0
$ echo 'x.y={aaa b .c}' | sed 's/[^[:alnum:]]\+//g'
xyaaabc
This is great, but I'd like to only modify the part within {}. So I thought this may need capture groups, hence I went ahead and tried these:
Trial 1
$ echo 'x.y={aaa b .c}' | sed -E 's/x.y=\{(.*)\}/x.y={\1}/'
x.y={aaa b .c}
Here I have captured the content I want to modify (aaa b .c) correctly, but I need a way to somehow do s/[^[:alnum:]]\+//g only on \1.
Instead, I tried capturing all alphanumeric characters only (to \1) like this:
Trial 2
$ echo 'x.y={aaa b .c}' | sed -E 's/x.y=\{([[:alnum:]]+)\}/x.y={\1}/'
x.y={aaa b .c}
Of course, it doesn't work because I'm only expecting alnum's and then immediately a } literal. I didn't tell it to ignore the non-alnum's. I.e, this part:
s/x.y=\{([[:alnum:]]+)\}/x.y={\1}/
^^^^^^^^^^^^^^^^^^
It literally matches: an open brace, some alnum's, and a closing brace -- which is not what I want. I'd like it to match everything, but only capture the alnum's.
Example of input/output:
x.y={aaa b .c} blah
blah
x.y={1 2 3 def} blah
blah
to
x.y={aaabc} blah
blah
x.y={123def} blah
blah
I searched the web before finally giving up and posting the question but I didn't find anything helpful as I didn't see anyone with a similar problem as mine. Would appreciate some help this as I'd love to have a better understanding of variables in regex/sed, thanks!

Answer 1

With your shown samples, please try following in awk. Written and tested in GNU awk.
awk '
match($0,/\{[^}]*}/){
val=substr($0,RSTART,RLENGTH)
gsub(/[^{}a-zA-Z]/,"",val)
$0=substr($0,1,RSTART-1) val substr($0,RSTART+RLENGTH)
}
1
' Input_file
Explanation: Adding detailed explanation for above.
awk ' ##Starting awk program from here.
match($0,/\{[^}]*}/){ ##using match function of awk to match from { to first occurrence of }
val=substr($0,RSTART,RLENGTH) ##Creating val which has sub string of matched regex in it.
gsub(/[^{}a-zA-Z]/,"",val) ##Globally substituting everything apart from { } and alphabets in val.
$0=substr($0,1,RSTART-1) val substr($0,RSTART+RLENGTH) ##saving everything before match val and everything after match here.
}
1 ##Printing line if it doesn't meet `match` condition mentioned above.
' Input_file ##Mentioning Input_file name here.
Generic solution: In case you have multiple occurrences of { and } then try following awk code.
awk '
{
line=""
while(match($0,/\{[^}]*}/)){
val=substr($0,RSTART,RLENGTH)
gsub(/[^{}a-zA-Z]/,"",val)
line=(line?line:"") (substr($0,1,RSTART-1) val)
$0=substr($0,RSTART+RLENGTH)
}
if(RSTART+RLENGTH!=length($0)){
$0=line $0
}
else{
$0=line
}
}
1
' Input_file

Answer 2

With sed (tested on GNU sed, syntax may vary for other implementations):
$ sed -E ':a s/(\{[[:alnum:]]*)[^[:alnum:]]+([^}]*})/\1\2/; ta' ip.txt
x.y={aaabc} blah
blah
x.y={123def} blah
blah
:a marks that location as label a (used to jump using ta as long as the substitution succeeds)
(\{[[:alnum:]]*) matches { followed by zero or more alnum characaters
[^[:alnum:]]+ matches one or more non-alnum characters
([^}]*}) matches till the next } character
If perl is okay:
$ perl -pe 's/\{\K[^}]+(?=\})/$&=~s|[^a-z\d]+||gir/e' ip.txt
x.y={aaabc} blah
blah
x.y={123def} blah
blah
\{\K[^}]+(?=\}) match sequence of { to } (assuming } cannot occur in between)
\{\K and (?=\}) are used to avoid the braces from being part of the matched portion
e flag allows you to use Perl code in replacement portion, in this case another substitute command
$&=~s|[^a-z\d]+||gir here, $& refers to entire matched portion, gi flags are used for global/case-insensitive and r flag is used to return the value of this substitution instead of modifying $&
[^a-z\d]+ matches non-alphanumeric characters (assuming ASCII, you can also use [^[:alnum:]]+)
use \W+ if you want to preserve underscores as well
For both solutions, you can add x\.y= prefix if needed to narrow the scope of matching.

Answer 3

Here is another gnu-awk solution using FPAT:
s='x.y={aaa b .c}'
awk -v OFS= -v FPAT='{[^}]+}|[^{}]+' '
{
for (i=1; i<=NF; ++i)
if ($i ~ /^{/) $i = "{" gensub(/[^[:alnum:]]+/, "", "g", $i) "}"
} 1' <<< "$s"
x.y={aaabc}