How to find information inside a xml tag using grep?

I can't see why you'd want to use grep for this, while it can be solved with a trivial XPath expression:

//title/text()

There are many command line tools for XPath and they're usually bundled with the OS.

Answers to this question on Stack Overflow list a number of such tools.

The problem with grep here is that it's a generic tool for text processing and it's not aware of any XML structure. For a very simple scenario, you can get it working. If the document is complex or if you're using this in a script that will survive months or years and not just a one-off job, you may end up feeling sorry for the results.

XPath makes it easy to tell the difference between similarly named tags that appear in different contexts in a document.

<article>
    <author>
        <name>Jon Doe</name>
        <title>Chief Editor</title>
    </author>
    <title>On the Benefits of grep</title>
    <publicationDate>2018-02-12</publicationDate>
    <text>blah blah blah</text>
</article>

Extracting the title of the article represented by this document with grep would fail if you used any of the other answers posted here. You could technically write the regular expression to get what you need but it's a lot easier with XPath.

/article/title/text()

If you know you're dealing with a trivial document and the format doesn't change or if it's a one time job where you can quickly validate the results, you can go for grep as explained by others.

Since you already use grep -P, why don't you use its features?

grep -oP '(?<=<title>).*?(?=</title>)'

In the general case, XPath is the correct solution, but for toy scenarios, yes Virginia, it can be done.