compare between strings

[altenbach]

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@Shva wrote:
I would like to compare string in two files and extract the word which is same in other one file.

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To do this, we need a few clarifications:

1. What is a "word". (Probably a consecutive string of readable characters, separated by one or multiple spaces, tabs, newlines, returns, etc.)?  You need a complete list of all word separators.
2. Is the "other file" just a list of keywords or is it also a long, arbitrary text file?
3. What should happen if more than one word matches (get the first match, get all matches, etc)?
4. What is "extract"? Do you want to delete the matching word form file 1 or just know which word matched?

Could you give an example?