Search and replace in bash using regular expressions

These examples also work in bash no need to use sed:

#!/bin/bash
MYVAR=ho02123ware38384you443d34o3434ingtod38384day
MYVAR=${MYVAR//[a-zA-Z]/X} 
echo ${MYVAR//[0-9]/N}

you can also use the character class bracket expressions

#!/bin/bash
MYVAR=ho02123ware38384you443d34o3434ingtod38384day
MYVAR=${MYVAR//[[:alpha:]]/X} 
echo ${MYVAR//[[:digit:]]/N}

output

XXNNNNNXXXXNNNNNXXXNNNXNNXNNNNXXXXXXNNNNNXXX

What @Lanaru wanted to know however, if I understand the question correctly, is why the "full" or PCRE extensions \s\S\w\W\d\D etc don't work as supported in php ruby python etc. These extensions are from Perl-compatible regular expressions (PCRE) and may not be compatible with other forms of shell based regular expressions.

These don't work:

#!/bin/bash
hello=ho02123ware38384you443d34o3434ingtod38384day
echo ${hello//\d/}


#!/bin/bash
hello=ho02123ware38384you443d34o3434ingtod38384day
echo $hello | sed 's/\d//g'

output with all literal "d" characters removed

ho02123ware38384you44334o3434ingto38384ay

but the following does work as expected

#!/bin/bash
hello=ho02123ware38384you443d34o3434ingtod38384day
echo $hello | perl -pe 's/\d//g'

output

howareyoudoingtodday

Hope that clarifies things a bit more but if you are not confused yet why don't you try this on Mac OS X which has the REG_ENHANCED flag enabled:

#!/bin/bash
MYVAR=ho02123ware38384you443d34o3434ingtod38384day;
echo $MYVAR | grep -o -E '\d'

On most flavours of *nix you will only see the following output:

d
d
d

nJoy!

Use sed:

MYVAR=ho02123ware38384you443d34o3434ingtod38384day
echo "$MYVAR" | sed -e 's/[a-zA-Z]/X/g' -e 's/[0-9]/N/g'
# prints XXNNNNNXXXXNNNNNXXXNNNXNNXNNNNXXXXXXNNNNNXXX

Note that the subsequent -e's are processed in order. Also, the g flag for the expression will match all occurrences in the input.

You can also pick your favorite tool using this method, i.e. perl, awk, e.g.:

echo "$MYVAR" | perl -pe 's/[a-zA-Z]/X/g and s/[0-9]/N/g'

This may allow you to do more creative matches... For example, in the snip above, the numeric replacement would not be used unless there was a match on the first expression (due to lazy and evaluation). And of course, you have the full language support of Perl to do your bidding...

This actually can be done in pure bash:

hello=ho02123ware38384you443d34o3434ingtod38384day
re='(.*)[0-9]+(.*)'
while [[ $hello =~ $re ]]; do
  hello=${BASH_REMATCH[1]}${BASH_REMATCH[2]}
done
echo "$hello"

...yields...

howareyoudoingtodday