Addition with 'sed'

Not sure if the 'e' flag for the 's' command has been discussed yet but here's how I'd solve this problem

$ echo 12 | sed -E 's/[0-9]*/expr & + 3/e'
15

the 'e' flag takes the current pattern line and runs it as a command before replacing the pattern space with the output. In this case using the 'expr' command to do our math. source: The S Command

A little more complicated, but converts multiple digits that end in a space from bytes->kilobytes (avoids numbers with a percent sign)

$ df | sed -E '1!{s/(b)([0-9]+)(s)/1$(expr 2 / 1024)kb3/g};s/.*/printf "%s" "&"/e'
Filesystem    512-blocks      Used Available Capacity iused      ifree %iused  Mounted on
/dev/disk1s5   478144kb  21563kb 141089kb    14%  477kb 2390246kb    0%   /
devfs                0kb       0kb         0kb   100%     0kb          0kb  100%   /dev
/dev/disk1s1   478144kb 297544kb 141089kb    68% 1727kb 2388995kb    0%   /System/Volumes/Data
/dev/disk1s4   478144kb  16384kb 141089kb    11%       0kb 2390723kb    0%   /private/var/vm
map auto_home          0kb         0kb         0kb   100%       0kb          0kb  100%   /System/Volumes/Data/home
/dev/disk2s1     1188kb   987kb    201kb    84%       0kb 4194303kb    0%   /Volumes/RecoveryHDMeta
/dev/disk3s1     3822kb   2528kb   1294kb    67%   46kb 4194257kb    0%   /Volumes/macOS Base System

1. wraps each number in '$(' & ')' the '1!{' & '}' makes sure that this ignores the first line

1!{s/(b)([0-9]+)(s)/1$(expr 2 / 1024)kb3/g};

2. matches whole line & prefixes line with the printf command, the e flag will then execute each line

;s/.*/printf "%s" "&"/e'

Although using sed expression is great it has its limitations. For example following fails:

$ echo "1000000000000000000000000000000+1" | sed -e 's/([0-9]*)+([0-9]*)/expr 1 + 2/e'
expr: 1000000000000000000000000000000: Numerical result out of range

To overcome this limitation one my simply turn to the built in power of pure sed and implement following arbitrary length decimal adder:

#!/bin/sed -f

s/+/n/g
s/$/nn0/

:LOOP
s/^(.*)(.)n(.*)(.)n(.*)n(.)$/01n03n5n624/
h
s/^.*n.*n.*n(...)$/1/

# decimal full adder module
# INPUT:  3digits (Carry in, A, B,)
# OUTPUT: 2bits (Carry, Sum)
s/$/;000=00001=01002=02003=03004=04005=05006=06007=07008=08009=09010=01011=02012=03013=04014=05015=06016=07017=08018=09019=10020=02021=03022=04023=05024=06025=07026=08027=09028=10029=11030=03031=04032=05033=06034=07035=08036=09037=10038=11039=12040=04041=05042=06043=07044=08045=09046=10047=11048=12049=13050=05051=06052=07053=08054=09055=10056=11057=12058=13059=14060=06061=07062=08063=09064=10065=11066=12067=13068=14069=15070=07071=08072=09073=10074=11075=12076=13077=14078=15079=16080=08081=09082=10083=11084=12085=13086=14087=15088=16089=17090=09091=10092=11093=12094=13095=14096=15097=16098=17099=18100=01101=02102=03103=04104=05105=06106=07107=08108=09109=10110=02111=03112=04113=05114=06115=07116=08117=09118=10119=11120=03121=04122=05123=06124=07125=08126=09127=10128=11129=12130=04131=05132=06133=07134=08135=09136=10137=11138=12139=13140=05141=06142=07143=08144=09145=10146=11147=12148=13149=14150=06151=07152=08153=09154=10155=11156=12157=13158=14159=15160=07161=08162=09163=10164=11165=12166=13167=14168=15169=16170=08171=09172=10173=11174=12175=13176=14177=15178=16179=17180=09181=10182=11183=12184=13185=14186=15187=16188=17189=18190=10191=11192=12193=13194=14195=15196=16197=17198=18199=19/
s/^(...)[^;]*;[^;]*1=(..).*/2/
H
g
s/^(.*)n(.*)n(.*)n...n(.)(.)$/1n2n53n4/
/^([0]*)n([0]*)n/ {
        s/^.*n.*n(.*)n(.)/21/
        s/^0(.*)/1/
        q
}
b LOOP

The way it works is by implementing decimal adder module that adds two input digits (A and B) as well as Carry Bit and produces a Sum and Carry bit. The idea is borrowed from electronic where binary adder does the same for binary numbers. All we have to do is loop the adder over all digits and we can add arbitrary length numbers (limited by memory). Below is the adder in action:

./decAdder.sed
666666666666666666666666666666999999999999991111111112222+1100000000000000000000011111111111111111111111111111111111
1766666666666666666666677777778111111111111102222222223333

In exactly the same way one can implement binary (or any other base) adder. All you have to do is replace the line that starts with s/$/;000=00001... with proper substitution pattern for given base. For example: s/$/;000=00001=01010=01011=10100=01101=10110=10111=11/ is substitution pattern for arbitrary length binary adder.

You can fit the code documented on my github.

Here's a Perl solution:

echo 12 | perl -wlpe '$_ += 3'
# Output:  15

If you prefer to change the first set of digits encountered in a string, you can use:

echo I am 12 years old. | perl -wlpe 's/(d+)/$1 + 3/e'
# Output:  I am 15 years old.

If you prefer to change all the sets of digits in a string, you can use the /g modifier, like this:

echo They are 11, 12, and 13 years old. | perl -wlpe 's/(d+)/$1 + 3/eg'
# Output:  They are 14, 15, and 16 years old.

I really don't get why the extreme complexity of the accepted answer, either of the below do what you want:

echo 12 | sed 's/[0-9]*/echo $(( & + 3 ))/e'

or

echo 12 | sed 's/[0-9]*/expr & + 3/e'

I think it might require GNU sed, but I am not sure.

I tried to accept your challenge @Richter, this is what I did using part of your code:

sed 's/[0-9]/<&/g
s/0//g; s/1/|/g; s/2/||/g; s/3/|||/g; s/4/||||/g; s/5/|||||/g; s/6/||||||/g
s/7/|||||||/g; s/8/||||||||/g; s/9/|||||||||/g
: tens
s/|</<||||||||||/g
t tens
s/<//g
s/.**$/0/
s/^*.*/0/
s/*|/*/
: mult
s/(|*)*|/1<1*/ 
t mult
s/*//g
s/<//g
: back
s/||||||||||/</g
s/<([0-9]*)$/<01/
s/|||||||||/9/; s/||||||||/8/; s/|||||||/7/; s/||||||/6/; s/|||||/5/; s/||||/4/
s/|||/3/; s/||/2/; s/|/1/
s/</|/g
t back'

Input:

04*3
4*3
40*3
42*32
150*20
1*3
3*1
0*3
3*0

Output: all the correct results

If you mix in some bashism:

echo $(($(echo 12 | sed 's/[0-9]*/&+3/')))

To extract the number from a text:

echo $(($(echo "foo12bar" | sed -r 's/[^0-9]*([0-9]*).*/1+3/')))

Without sed, just bash:

var="foo12bar"
echo $((${var//[^0-9]/}+3))

replaces every non-digit ${var//[^0-9]/} and does arithmetic in double round parens: $((x+3))

just feed the string into a calculator

 echo 12 | sed 's/[0-9]*/&+3/' | bc

perl allows for a very similar construct to sed's ... one difference is that perl can do more complex things... sed is very good for simple text substitions

 echo 'a12' | perl -pe 's/([0-9]+)/($1+3)/e'  # the trailing /e means evaluate

output

a15

If you definitely have to combine regular expressions and arithmetic operations, choose a language where the regular expression's replacement parameter can be a callback function.

Perl, Ruby, JavaScript and Python are such languages:

bash-4.2$ echo 12 | perl -pe 's/d+/$&+3/e'
15

bash-4.2$ echo 12 | ruby -pe '$_.sub!(/d+/){|s|s.to_i+3}'
15

bash-4.2$ echo 12 | js -e 'print(readline().replace(/d+/,function(s){return parseInt(s)+3}))'
15

bash-4.2$ echo 12 | python -c 'import re;print re.sub("d+",lambda s:str(int(s.group(0))+3),raw_input())'
15

Another simple bash solution, that actually works in a pipe:

 echo 12 | { read num; echo $(( num + 3)); }

sed isn't the best option here, it doesn't do arithmetics natively (see Increment a number for how you could possibly do it though). You could do that with awk:

$ echo 12 | awk '{print $0+3}'
15

The best piece of code to use will depend on the exact format of your input and what you want/need to do if it is not numeric, or contains more than one number, etc.

You could also do this only with bash:

$ echo $(( $(echo 12) + 3 ))

or using expr in a similar fashion.