Are there limited number of words we can append a 're to?
Are the following words correct:
In the American English that I'm familiar with, native speakers do not typically say Where you going? To my ear, it is usually pronounced with an extra beat, making it Where r you going? which I would represent in writing with the contraction where're. This is in contrast to the more formal where are.
In English, are contracts to 're. Technically, you can use a contraction wherever you might normally use the full, written out word. However, you should be very careful when doing this. Contractions are:
Informal : While contractions can be very useful in written English, many experts caution against the use of contractions in formal communication. Since contractions tend to add a light and informal tone to your writing, they are often inappropriate for academic research papers, business presentations, and other types of official correspondence.
Sometimes hard to distinguish from other words in the sentence: Saying "Where're" out loud leads to something along the lines of where-er. As "where're" is not a common contraction, many people will have trouble understanding.
Used more for speaking: Since contractions are, as mentioned above, informal, they tend to be used more in speech. Because of this, you may not want to use "here're" and "where're" because they are hard to pronounce and differentiate from other words in the sentence.
In short, you can use those contractions, and you can technically affix "'re" to any word which is followed by are. However, use caution for the reasons above.
The written contraction 're is appropriate where the spoken form that is being represented has contracted the word are into an /r/ sound attached to the preceding syllable. For instance, they're represents a pronunciation that is the same as, or very close to, the pronunciation of there and their.
In a case like a spoken sequence like Where are you going? in which the word are is not fully enunciated, you'll nonetheless notice the /r/ sound does not attach to the preceding syllable (where), but has its own vowel sound. Hence Where are you going? is the appropriate written form. (Note it is common to speak without the word are at all; to represent that, where you going? would be right).
Contractions in general in formal writing isn't (tiny joke for those who appreciate irony) recommended, and few (compared with their use when speaking) in informal writing help the reader (IMHO). Specifically with the 're contraction: even when trying to report spoken words, what sounds to be 're is best written "are" (as mgkrebbs posted), and saying where're or here're can sound too much like where or here, as simchona posted), but there is another concern with "where're":
"Where e're" is used in poetry and songs for: "Where ever", but it can very easily be confused with "Where're" in speaking BUT ALSO in writing, as a quick google search showed me. So even more reason to avoid "where're"!