How do I make a wildcard package search using apt-get?

If you wish to search a specific string only in package names, you can do:

apt-cache search --names-only 'xxx'

This will return anything that contains 'xxx' in their package name.

You can also use regular expressions:

apt-cache search --names-only 'php5$'

This will return any package with their name ending in php5.

Regular expressions can also be used without the --names-only. See apt-cache(8) man page.