CSS word wrap / line break on underscores in addition to whitespace and hyphens

You can use the <wbr> tag (“Line Break Opportunity element”), which lets the browser break the line wherever you place it.

So your HTML should be:

Here_<wbr>is_<wbr>an_<wbr>example_<wbr>of_<wbr>a_<wbr>really_<wbr>...

You can add this tag on the server-side before you output the HTML.

An alternative is the entity ​ which is a zero width space. Sometimes this works better on certain browsers.