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Match vocabulary words and phrases

I am writing an application/logic that has vocabulary word/phrase as an input parameter. I am having troubles writing validation logic for this parameter's value!
Following are the rules I've came up with:
can be up to 4 words (with hyphens or not)
one apostrophe is allowed
only regular letters are allowed (no special characters like !##$%^&*()={}[]"";|/>/? ¶ © etc)
numbers are disallowed
case insensitive
multiple languages support (English, Russian, Norwegian, etc..) (so both Unicode and Cyrillic must be supported)
either whole string matches or nothing
Few examples (in 3 languages):
// match:
one two three four
one-two-three-four
one-two-three four
vær så snill
тест регекс
re-read
under the hood
ONe
rabbit's lair
// not-match:
one two three four five
one two three four#
one-two-three-four five
rabbit"s lair
one' two's
one1
1900
Given the expected result provided above - could someone point me to right direction on how to create a validation rule like that? If that matters - I will be writing validation logic in C# so I have more tools than just Regex available at my disposal.
If that is going to be of any help - I have been testing several solutions, like these ^[\p{Ll}\p{Lt}]+$ and (?=\S*['-])([a-zA-Z'-]+)$. The first regex seems to be doing a great job allowing just the letters I need (En, No and Rus), whereas the second rule set is doing great in using the Lookahead concept.
\p{Ll} or \p{Lowercase_Letter}: a lowercase letter that has an uppercase variant.
\p{Lu} or \p{Uppercase_Letter}: an uppercase letter that has a lowercase variant.
\p{Lt} or \p{Titlecase_Letter}: a letter that appears at the start of a word when only the first letter of the word is capitalized.
\p{L&} or \p{Letter&}: a letter that exists in lowercase and uppercase variants (combination of Ll, Lu and Lt).
\p{Lm} or \p{Modifier_Letter}: a special character that is used like a letter.
\p{Lo} or \p{Other_Letter}: a letter or ideograph that does not have lowercase and uppercase variants.
Needless to say, neither of the solutions I have been testing take into account all the rules I defined above..

You can use
\A(?!(?:[^']*'){2})\p{L}+(?:[\s'-]\p{L}+){0,3}\z
See the regex demo. Details:
\A - start of string
(?!(?:[^']*'){2}) - the string cannot contain two apostrophes
\p{L}+ - one or more Unicode letters
(?:[\s'-]\p{L}+){0,3} - zero to three occurrences of
[\s'-] - a whitespace, ' or - char
\p{L}+ - one or more Unicode letters
\z - the very end of string.
In C#, you can use it as
var IsValid = Regex.IsMatch(text, #"\A(?!(?:[^']*'){2})\p{L}+(?:[\s'-]\p{L}+");{0,3}\z")

Regex to match all alphanumeric and math operators

I have the simple regex #"[a-zA-Z]" to match all characters a-z in a string but I also need math operators (*, /, +, -). I was reading over the documentation on msdn but I got lost relatively fast due to the math operators being used as other tokens in the regex
This solution works:
#"[A-Za-z\*\+\-\/]"
Thanks for the help and resources everyone.

The correct answer is
#"[A-Za-z*+/-]"
Or #"[A-Za-z-*+/]", or #"[-A-Za-z*+/]", or #"[A-Za-z*\-+/]".
Or, shorten it with a case-insensitive modifier: #"(?i)[A-Z*+/-]" (or use a corresponding RegexOptions.IgnoreCase with #"[A-Z*+/-]" since it seems you are using C#).
Inside a character class, the unescaped hyphen should either be at the start or final position to be treated as a literal, or right after a range or shorthand class. Otherwise, it must be escaped. Also, ] must be escaped if not at the beginning of the character class. Other characters do not have to be escpaed inside a character class.
To test, use an appropriate online regex tester. You need one for .NET, see Regex demo at RegexStorm.

Foreign language characters in Regular expression in C#

In C# code, I am trying to pass chinese characters: " 中文ABC123".
When I use alphanumeric in general using "^[a-zA-Z0-9\s]+$",
it doesn't pass for "中文ABC123" and regex validation fails.
What other expressions do I need to add for C#?

To match any letter character from any language use:
\p{L}
If you also want to match numbers:
[\p{L}\p{Nd}]+
\p{L} ... matches a character of the unicode category letter.
it is the short form for [\p{Ll}\p{Lu}\p{Lt}\p{Lm}\p{Lo}]
\p{Ll} ... matches lowercase letters. (abc)
\p{Lu} ... matches uppercase letters. (ABC)
\p{Lt} ... matches titlecase letters.
\p{Lm} ... matches modifier letters.
\p{Lo} ... matches letters without case. (中文)
\p{Nd} ... matches a character of the unicode category decimal digit.
Just replace: ^[a-zA-Z0-9\s]+$ with ^[\p{L}0-9\s]+$

Thanks to #Andie2302 for pointing to the right way to do it.
In Addition, for many language in the world, it's still has the 'addition character' that require main character to generate it (ex. Thai word 'เก็บ' if use only \p{L} it will display only 'เกบ', you can see that some symbolic will be missing from the word).
That's why only \p{L} will not work for all foreign language.
So, you need to use code below, to support almost foreign language
\p{L}\p{M}
NOTE:
L stand for 'Letter' (All letter from all language, but does not include the 'Mark')
M stand for 'Mark' (The 'Mark' cannot display alone, it require 'Letter' to display it)
In Addition that you need Number, use code below
\p{N}
NOTE:
N stand for 'Numeric'
Thanks to this website for very useful information
https://www.regular-expressions.info/unicode.html

Regex ignore underscores

I have a regex ([-#.\/,':\w]*[\w])* and it matches all words within a text (including punctuated words like I.B.M), but I want to make it exclude underscores and I can't seem to figure out how to do it... I tried adding ^[_] (e.g. (^[_][-#.\/,':\w]*[\w])*) but it just breaks up all the words into letters. I want to preserve the word matching, but I don't want to have words with underscores in them, nor words that are entirely made up of underscores.
Whats the proper way to do this?
P.S.
My app is written in C# (if that makes any difference).
I can't use A-Za-z0-9 because I have to match words regardless of the language (could be Chinese, Russian, Japanese, German, English).
Update
Here is an example:
"I.B.M should be parsed as one word w_o_r_d! Russian should work too: мплекс исторических событий."
The matches should be:
I.B.M.
should
be
parsed
as
one
word
Russian
should
work
too
мплекс
исторических
событий
Note that w_o_r_d should not get matched.

Try this instead:
([-#.\/,':\p{L}\p{Nd}]*[\p{L}\p{Nd}])*
The \w class is composed of [\p{L}\p{Nd}\p{Pc}] when you're performing Unicode matching. (Or simply [a-zA-Z0-9] if you're doing non-Unicode matching.)
It's the \p{Pc} Unicode category -- punctuation/connector -- that causes the problem by matching underscores, so we explicitly match against the other categories without including that one.
(Further information here, "Character Classes: Word Character", and here, "Character Classes: Supported Unicode General Categories".)

Tue underscore comes from \w.
Simply use A-Za-z0-9 instead.

For a more concise version of LukeH's regex, you can use simply:
([-#.\/,':\p{L}]*\p{L})*
I simply used \p{L} instead of Lu, Ll, Lt, Lo, Lm. See Supported Unicode General Categories

Regular expression to catch letters beyond a-z

A normal regexp to allow letters only would be "[a-zA-Z]" but I'm from, Sweden so I would have to change that into "[a-zåäöA-ZÅÄÖ]". But suppose I don't know what letters are used in the alphabet.
Is there a way to automatically know what chars are are valid in a given locale/language or should I just make a blacklist of chars that I (think I) know I don't want?

You can use \pL to match any 'letter', which will support all letters in all languages. You can narrow it down to specific languages using 'named blocks'. More information can be found on the Character Classes documentation on MSDN.
My recommendation would be to put the regular expression (or at least the "letter" part) into a localised resource, which you can then pull out based on the current locale and form into the larger pattern.

What about \p{name} ?
Matches any character in the named character class specified by {name}.
Supported names are Unicode groups and block ranges. For example, Ll, Nd, Z,
IsGreek, IsBoxDrawing.
I don't know enough about unicode, but maybe your characters fit a unicode class?

See character categories selection with \p and \w unicode semantics.

All chars are "valid," so I think you're really asking for chars that are "generally considered to be letters" in a locale.
The Unicode specification has some guidelines, but in general the answer is "no," you would need to list the characters you decide are "letters."

Is there a way to automatically know what chars are are valid in a given locale/language or should I just make a blacklist of chars that I (think I) know I don't want?
This is not, in general, possible.
After all Engligh text does include some accented characters (e.g. in "fête" and "naïve" -- which in UK-English to be strictly correct still use accents). In some languages some of the standard letters are rarely used (e.g. y-diaeresis in French).
Then consider including foreign words are included (this will often be the case where technical terms are used). Quotations would be another source.
If your requirements are sufficiently narrowly defined you may be able to create a definition, but this requires linguistic experience in that language.

This regex allows only valid symbols through:
[a-zA-ZÀ-ÿ ]