<p>For example, this regex</p>

<pre><code>(.*)&lt;FooBar&gt;
</code></pre>

<p>will match:</p>

<pre><code>abcde&lt;FooBar&gt;
</code></pre>

<p>But how do I get it to match across multiple lines?</p>

<pre><code>abcde
fghij&lt;FooBar&gt;
</code></pre>

In the context of use within languages, regular expressions act on strings, not lines. So you should be able to use the regex normally, assuming that the input string has multiple lines.
In this case, the given regex will match the entire string, since "<FooBar>" is present. Depending on the specifics of the regex implementation, the $1 value (obtained from the "(.*)") will either be "fghij" or "abcde\nfghij". As others have said, some implementations allow you to control whether the "." will match the newline, giving you the choice.
Line-based regular expression use is usually for command line things like egrep.