Various punctuation characters have a noticeable impact on search results - mostly from a searcher perspective. As a webmaster, you may find that your users include punctuation in some keywords, and so it can be of use to know what the effect on the results they see is. And besides, knowing how to search Google is one step towards understanding how Google works. This is a spot check of the current handling of punctuation by Google.
Underscores are treated as a letter of the alphabet, which is why you can search for an underscore directly [google.com]. Use underscores in content if your visitors include an underscore when searching (e.g. if you had a programming site).
Ampersands or 'and symbols' have fairly unique handling. They're both indexed [google.com] and also treated as the equivalent of word "and". If there are no spaces separating the symbol and the adjacent letters, the search results are an approximate equivalent of combining results for ["key and word"] and ["key & word"] (note the phrase matching). Use ampersands in copy as is natural for your target audience.
Explicit search operators
Many punctuation characters are explicit search operators, with a documented effect on results. Search operators are not indexed (or at least, they can't be searched for) and so are usually treated as word separators when found within website copy:
An (unbroken) pipe character is the equivalent of boolean OR: a search for [key OR word]. It can be a handy shortcut when conducting complex queries.
A double quote triggers an exact or phrase search for the proceeding words (whether you include a closing double quote or not). So in this instance, it's the equivalent of a search for [key word] since a single word can't be a phrase. ["key word] is the same as searching for ["key word"].
An asterisk is a wildcard search for zero or more words: [key ... word]. Putting numbers on both sides will trigger the calculator. Occasionally, Google delivers (strange!) results if you search for an asterisk directly.
A tilde triggers Google's related word operator - in this instance, a search for both 'key' and 'word', as well as other words related to 'word' - like 'Microsoft', 'dictionary' and others.
Search operator oddities
A hyphen (as is probably consistent with language use) returns a mix of results for the words both used separately, and joined together - somewhere between [key word] and [keyword]. It's the preferred word separator within website URLs, since other punctuation characters that are treated as a word-separator have specific functions within a URL.
A few punctuation characters have a strange impact on results - returning far fewer results than for either separated or concatenated words. They are neither known search operators, or indexed characters. These are . / \ @ = :
As far as I'm, aware, all other punctuation characters are treated as simply a space or word separator.
So, do I have too much time on my hands? Probably. But why not confuse whoever looks at Google's search logs by
trying a few punctuation searches yourself? ;)
Do you know any punctuation with an effect on results not discussed here, or more about the effect on results of the punctuation above?