How to get multiple lines out of a file by a regex?

How to get numerous lines out of a documents by a regex?

I usually would love to get numerous lines / change numerous lines by a regex. An instance case:

I am attempting to read component of an XML/SGML documents (they are not always well created or in a foreseeable syntax, so a regex would certainly be more secure than a correct parser. On top of that I would love to have the ability to do this additionally whit entirely disorganized files where simply some keywords are recognized.) in a shell script (working on Solaris and also Linux).

Instance XML:

<tag1>
 <tag2>bar</tag2>
</tag1>
<tag1>
 <tag2>foo</tag2>
</tag1>

From this I would love to read the <tag1> if it has foo someplace within it.

A regex like (<tag1>.*?foo.*?</tag1>) need to offer the appropriate component yet devices like grep and also sed just benefit me with solitary lines. Just how can I get

<tag1>
 <tag2>foo</tag2>
</tag1>

in this instance?