I'm trying to find and replace text in hundreds of files,

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I'm trying to find and replace text in hundreds of files, but there's a special requirement.

For example, let's say the text is as follows :

test
40
blab

I want to replace the 40 with another string, but i want to avoid replacing files with text that looks like this :

test
required:40
blab

If i just find and replace the number "40",i will end up replacing a lot of stuff that i don't want to. So basically want to replace the number 40 if it's the ONLY thing by itself on a single line.

I've tried several find and replace programs and none can seem to do it. Any ideas?