how would you get SED to do the following, say you have the following lines in a text file:

user=tigger
some text some text
some text some text
some text some text
user=ted
some text some text
some text some text
some text some text
user=thekingofrockandroll

you want to find any line which has 'user=[the user name]' and replace it with 'user=xxx'

so the string after the 'user=' will always contain a different number of characters but you just want to sed to find any lines with 'user=' and replace the part after 'user=' with 'xxx' no matter what?

and...

if what to replace say 'AM' in a text file with 'JO', i can do:

sed 's/AM/JO/g;s/am/jo/g'

sed 's/user=.*/user=xxx/'

sed 's/^am$/jo/i'

hmm, that's not working the 'am' is still present in the file after it has ran? I have:

sed 's/^am$/jo/i' "$f" > "$f".tmp && cp -f "$f".tmp "$f" && rm "$f".tmp

sed 's/^am/jo/i' "$f" > "$f".tmp && cp -f "$f".tmp "$f" && rm "$f".tmp

^ means start of the line, matching regardles of the characters' case is done by /i modifier.

hmm, what I would like ideally is this (say I'm replacing JO with BO)

my file to run sed on looks like this (as an example):

akdjfdkl sdjfklsjdf jsdfkljsf JO KSJDFKDFJLJOdsjfskldjf dsjfkljsfl <---- first JO on this line should be replaced by sed, second one should not because it forms part of a word
akdjalsd@JO <--- JO should be replaced here by sed
sdfkjdslfjsl@dfdsf_JO <----- and here
JO <---- and here
jo <----- and here

Does it have to be sed?

perl -pe 's/(?<![a-z])jo(?![a-z])/bo/ig' file

Hell no!