How to detect end of line with sed

With standard sed, you will never see a newline in the text read from a file. This is because sed reads line by line, and there is therefore no newline at the end of the text of the current line in sed's pattern space. In other words, sed reads newline-delimited data, and the delimiters are not part of what a sed script sees.

Regular expressions can be anchored at the end of the line using $ (or at the beginning, using ^). Anchoring an expression at the start/end of a line forces it to match exactly there, and not just anywhere on the line.

If you want to replace anything matching the pattern [A-Za-z]* at the end of the line with something, then anchoring the pattern like this:

[A-Za-z]*$

...will force it to match at the end of the line and nowhere else.

However, since [A-Za-z]*$ also matches nothing (for example, the empty string present at the end of every line), you need to force the matching of something, e.g. by specifying

[A-Za-z][A-Za-z]*$

or

[A-Za-z]\{1,\}$

So, your sed command line will thus be

$ sed 's/[A-Za-z]\{1,\}$/replace/' file.txt

I did not use the -E switch here because it's not needed. With it, you could have written

$ sed -E 's/[A-Za-z]+$/replace/' file.txt

It's a matter of taste.

sed "s/[a-zA-Z]*$/replace/" input.txt > result.txt

Or, the long complex unnecessary way:

I've found out, this can be done, still using sed, with the help of tr. You can assign another character to represent the end of the line. Another temporary character has to be used, in this case "`". Let's use "~" to represent the end of the line:

tr '\n' '`' <input.txt >output.txt
sed -i "s/`/~`/" output.txt
tr '`' '\n' <output.txt >result.txt

And then to perform the actual search and replace, use "~" rather than "\n":

sed -i -E "s/[a-zA-Z]*~/replace/" result.txt

And then clean up the extra character on the other lines:

sed -i "s/~//" result.txt

Obviously, this can all be piped together resulting in something like:

tr '\n' '`' <input.txt | sed -e "s/`/~`/" | tr '`' '\n' | sed -E -e "s/[a-zA-Z]*~/replace/" | sed "s/~//" > result.txt