The e modifier is a PHP-specific modifier that triggers PHP to run the resulting string as PHP code. It is basically a eval() wrapped inside a regex engine.
eval() on its own is considered a security risk and a performance problem; wrapping it inside a regex amplifies both those issues significantly.
It is therefore considered bad practice, and is being formally deprecated as of the soon-to-be-released PHP v5.5.
PHP has provided for several versions now an alternative solution in the form of preg_replace_callback(), which uses callback functions instead of using eval(). This is the recommended method of doing this kind of thing.
With specific regard to the code you've quoted:
I don't see an e modifier in the sample code you've given in the question. It has a slash at each end as the regex delimiter; the e would have to be outside of that, and it isn't. Therefore I don't think the code you've quoted is likely to be directly vulnerable to having an e modifier injected into it.
However, if $input contains any / characters, it will be vulnerable to being entirely broken (ie throwing an error due to invalid regex). The same would apply if it had anything else that made it an invalid regular expression.
Because of this, it is a bad idea to use an unvalidated user input string as part of a regex pattern - even if you are sure that it can't be hacked to use the e modifier, there's plenty of other mischief that could be achieved with it.
