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I have encountered several times the case when I want to search a part of the word, grep fails to find those matches. It works when I search full word.

For example, I need to find all files that contain "mysql_" word. Because I am dealing with legacy code that uses "mysql" instead of "mysqli" to work with a database, and want to see how much of the code uses "mysql".

Now, my command is 

grep -rnw './' -e "mysql_"

or

grep -P -rnw '\bmysql_' .

This still doesn't work.

But if I type:

grep -rnw './' -e "mysql_query"

then of course, grep finds all matches.

What am I doing wrong?

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Because you are using the '-w' parameter which means that your search pattern should occur as a full word (in other words, is delimited "non-word" characters):

 -w, --word-regexp Select only those lines containing matches that form whole words. The test is that the matching substring must either be at the beginning of the line, or preceded by a non-word constituent character. Similarly, it must be either at the end of the line or followed by a non-word constituent character. Word-constituent characters are letters, digits, and the underscore.

Note that in a regex, an underscore is a "word" character..

Since mysql_ only appears as part as a word (as in mysql_query), grep -w doesn't report it.

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