Shell Programming and Scripting

# val=9899
echo $INPUT|sed 's/\(._.\{8\}\).\{4\}\(_.*_\).*/\1'$val'\2/'
1_120606259899_3_4.5_

# echo $INPUT|sed 's/\(._.\{8\}\).\{4\}\(_.*\)_.*/\1'$val'\2/'
1_120606259899_3_4.5

._ --> one char (first char) (1)
.\{8\}\ --> 8 chars
\( ........ \)  --> save portion of our string for write in the RHS (cut and save it to buffer and it is equal to \1 [backreference numbered 1] , 
the sed structure is named "back reference headache(sed buffer) for after the write the stdoutput in the RHS[the right-hand side] 
liitle expl --> sed 's/THIS/CHANGE/'  --> LHS is the "THIS" in here and RHS is the "CHANGE")
.\{4\}  --> remaining 4 chars from our string

so we get the ._ -> first char from string --> "1" and "_"
and 8 chars --> "12060625"  
and 4 chars --> "9532"
\(._.\{8\}\) --> (\1 --> 1_12060625)
\(_.*\)  --> "_3_4.5"  (it is our back reference numbered 2 --> it expressed like --> \2)
_.*  --> "_888" 

and in the RHS(right-hand side sed 's/...../...../') we can the write that the our values

so ....../\1'$val'\2/
\1 --> 1_12060625
$val --> "9899"
\2 --> "_3_4.5"

result is "1_120606259899_3_4.5"