Unix: sed to change characters in particular position

Suppose you have big file. You want to change character 100-110 with some text. So, simply you want to interchange first 4 characters with the next 4 characters in each line. In these kind of scenarios, sed one liners will be very useful.

See few examples below:

$cat a.tmp
20090918ARPITH2010011634
20090918ARPITH2010012050
20090905ARPITH2010011382
20090824ARPITH2010012075
20090921ARPITH2010012075
--Follwing sed command will interchange first 4 digits with next 4 digits.
$sed 's:\([0-9]\{4\}\)\([0-9]\{4\}\):\2\1:' a.tmp
09182009ARPITH2010011634
09182009ARPITH2010012050
09052009ARPITH2010011382
08242009ARPITH2010012075
09212009ARPITH2010012075
Explanation:
\([0-9]\{4\}\) It defines first four digits. If you want to replace any 4 characters not only digit then replace [0-9] with . like \(.\{4\}\)
\ It is to escape (
() This bracket will define different groups. It is used for backreference and can be used again using \1 or \2 etc based on its occurence.
[0-9] Check for digits
\{4\} Four occurence of digit in [0-9]

\2\1 It says first put second set and then first set.

Following sed command will interchange first 8 characters/digits with next 6 characters. And it will also seperate each set with space.
$sed 's:\(.\{8\}\)\(.\{6\}\):\2 \1 :' a.tmp
ARPITH 20090918 2010011634
ARPITH 20090918 2010012050
ARPITH 20090905 2010011382
ARPITH 20090824 2010012075
ARPITH 20090921 2010012075

Following command will replace characters from 9 to 15 with text "NIRAVB"
sed 's:\(.\{8\}\)\(.\{6\}\):\1NIRAVB:' a.tmp
20090918NIRAVB2010011634
20090918NIRAVB2010012050
20090905NIRAVB2010011382
20090824NIRAVB2010012075
20090921NIRAVB2010012075

Limitation: If you specify more than 255 in curly braces "\{ \}", you will get following error.
$sed 's:\(.\{256\}\)\(.\{6\}\):\1NIRAVB:' a.tmp
sed: Function s:\(.\{256\}\)\(.\{6\}\):\1NIRAVB: cannot be parsed.

Solution: To overcome the issue you can break 335 in 255 + 80. As I have shown in following example. Here I want to change character 335 to 338 with text "TEST"
$cat a.tmp
20090913476178957893478958937589078903745897123890789074589076238904758906179038759081738904715890378904518978907348907579487592763785691287589072386590827890752890475890274890768902748906892748976898000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000005
20090913476178957893478958937589078903745897123890789074589076238904758906179038759081738904715890378904518978907348907579487592763785691287589072386590827890752890475890274890768902748906892748976898000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000005
20090913476178957893478958937589078903745897123890789074589076238904758906179038759081738904715890378904518978907348907579487592763785691287589072386590827890752890475890274890768902748906892748976898000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000005

$sed 's:\(.\{335\}\)\(.\{4\}\):\1TEST:' a.tmp
sed: Function s:\(.\{335\}\)\(.\{4\}\):\1TEST: cannot be parsed.

$sed 's:\(.\{255\}\)\(.\{80\}\)\(.\{4\}\):\1\2TEST:' a.tmp

20090913476178957893478958937589078903745897123890789074589076238904758906179038759081738904715890378904518978907348907579487592763785691287589072386590827890752890475890274890768902748906892748976898000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000TEST00000000000000000000000000000000005
20090913476178957893478958937589078903745897123890789074589076238904758906179038759081738904715890378904518978907348907579487592763785691287589072386590827890752890475890274890768902748906892748976898000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000TEST00000000000000000000000000000000005
20090913476178957893478958937589078903745897123890789074589076238904758906179038759081738904715890378904518978907348907579487592763785691287589072386590827890752890475890274890768902748906892748976898000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000TEST00000000000000000000000000000000005