split column after nth character

Question

I'm trying to split my second column in the file below after every 4 characters.

file.txt

>1A    THISISATEST
>1B    THATISATEST

desired output:

>1A    THIS    ISATEST
>1B    THAT    ISATEST

After searching and attempting to modify, I tried to use this sed command: sed 's/(.{4})(.{7}).*/\2 \3/' file.txt. However, I can't seem to get it to work. Am I missing something? However, if you have an awk suggestion, that would also be helpful. Also, please explain your suggestions. I'm in the learning process of awk and sed.

Answer 1

Here is a solution with awk. It separates first four characters and rest of the 2nd column into two variables and print them.

]$ awk '{s=substr($2,1,4)}{g=substr($2,5,length($2))}{print $1,s,g}' file.txt
1A THIS ISATEST
1B THAT ISATEST

Answer 2

With sed:

sed 's/^[^ ]\+ \+[^ ]\{4\}/&\t/'

How it works:

• ^: beginning of the line
• [^ ]\+: not space character matched at least one time
• \+: space character matched at least one time
• [^ ]\{4\}: not space character matched exactly four times
• &: everything matched between previous //
• \t: tab

Output:

>1A    THIS ISATEST
>1B    THAT ISATEST

---

Posixly correct:

sed 's/^[^ ][^ ]*  *[^ ]\{4\}/&\t/'

The pattern + is GNU extension, so one need to simulate it with repeated character and a * to be posix compatible.

Answer 3

sed 's/ [^ ]\{1,4\}/& /' <in >out

---

>1A    THIS ISATEST

---

It will work to insert a <space> after the longest match of the first occurrence of a sequence of 1 through 4 not-space characters immediately following a space character. This means that if the line starts with spaces it will affect the first not-space sequence (which is arguably correct behavior), or else if the second space-separated column consists of four or fewer characters it will append an additional space to that column (which is arguably incorrect behavior).

In no case does it substitute in a space for every fourth not-space character in the second column, though it does transform the example input to the example output.

Answer 4

With GNU awk

awk -F'\t' -vOFS='\t' '
  {patsplit($2, a, /.{4}/); 
   $2=""; for (k in a) $2=$2($2?"\t":"")a[k];  
   print}' file

Use patsplit to split the second field ($2) into 4 character chunks stored in array a. Join the chunks back with a tab separator and set into field 2 before printing out the record

1A    THIS  ISATEST
1B    THAT  ISATEST

Answer 5

With GNU awk you can use FPAT

awk '{$1=$1}1' OFS='\t' FPAT='>..\\s+\\w{4}|\\w*$'

• expression >..\\s+\\w{4} match to >1A THIS and \\w*$ match to the rest of string.

• $1=$1 forces to output string with new separator OFS

• 1 is substitute for {print $0}