I think, the key point answering this question would be by understanding the distinction between an alphabet and a script.
Some boring definitions
Alphabet is defined as a set of letters. Examples are Latin, Cyrillic, etc.
Script, or, more formally, Writing system, is a language-specific set of rules used to encode a text in a particular language.
For each language, script includes (a part of) a certain Alphabet, certain type of Numerals, Punctuation marks, and other symbols.
Now, let's get to the question
With regard to English, is there any other symbols that can compose a word?
English writing system includes:
- Latin letters, letter-case and capital-case, obviously;
- Arabic numerals, obviously;
- Diacritic marks used for loanwords;
- Punctuation symbols - make sure your software properly processes similarly looking symbols, e.g. apostrophes used to mark possession, and to mark contractions;
- For Natural Language Processing (NLP) software, you have to be aware that most characters with diacritic marks have their own Unicode points, so, for instance, parsing a sequence of
A and ◌̂ (a zero-width combining circumflex accent modifier, U+0302, not to be confused with a standalone character
^) should work exactly as parsing a single code point of
Â (Latin A with circumflex)
- If you are parsing Unicode text, you have to be also aware about ligatures, here's the list, and there may be stylistic ligatures as well;
Additionally, I would be interested in learning about other European languages and there use of such symbols within words which fall outside those defined by the alphabet.
Again, don't look into an alphabet. Look into writing system of a corresponding language.