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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?
You mentioned the asterisk as a wild card search. There's a seldom discussed approach for finding some long tail phrases.
Suppose [brandname product] is the search you'd really like to target, but the competition for that top phrase is a bit intense. What three word phrases might you be able to rank for? Try the search [brandname product *] and notice the other words that show up in bold text on the SERP.
The * wildcard search is also helpful for monitoring online reputation - [firstname lastname *] or [companyname *] can show you a lot sometimes.
I figured out the effect of the other punctuation too: key/word (and the others) returns results for ["key word"]. I originally thought they might be viewed as 'typo punctuation' i.e. the result of a mistype, but this is not the case.
For certain words that commonly contain punctuation, Google both indexes, and returns results for keywords containing punctuation. So, we get very different results when searching for C [google.com], C# [google.com], and C++ [google.com].
This also means that you can search for musical chords, and if you're doing well on your exams, an A+ [google.com]. In fact, trailing plus symbols always seem to return results.
In some instances, punctuation that would be useful for searches are not returned, for instances when searching for currency symbols, and in the case of dollar signs, programming variables.
I'm not clear on what the criteria are for punctuation to be considered a valid part of a search query.
Do it yourself and research what you see. No need to talk about it in public.
Common punctuation characters certainly seem to be indexed, but only included as part of the query if certain criteria are met. Those criteria do not appear to be based on frequency of search, but rather frequency of occurrence within the index (that's just pure speculation, I've not tested any data).
I would also consider Google's treating the possessive case (apostrophe + s or s + apostrophe) to be an odditie too.
Agreed: it seems to be of the same group, although also in the group that is considered part of the query if criteria are met.
pertains to content searched, not just how Google seems to interpret it
That's implicit in any keyword search (relevancy!). What punctuation does (or at least, what it is intended to do).
[edited by: Receptional_Andy at 10:28 pm (utc) on July 8, 2008]