Try this:

/^([a-zA-Z ]){2,30}$/

Try this:

/^(([A-Za-z]+[\-\']?)*([A-Za-z]+)?\s)+([A-Za-z]+[\-\']?)*([A-Za-z]+)?$/

It expects optionally [at least 1 alphabetical character followed by a ' or -] an indefinite number of times. There must be at least one alphabetical character before a required space to ensure we are getting at least the first and last name. This entire pattern is grouped to accept indefinite repetition (for people who like to use all their names, such as John Jacob Jingleheimer Schmidt), but must appear at least once, by means of the + sign right in the middle. Finally, the last name is treated the same way as the other names, but no trailing space is allowed. (Unfortunately this means we are violating DRY a little bit.)

Here is the outcome on several possible pieces of input:

"Jon Doe": true
"Jonathan Taylor Thomas": true
"Julia Louis-Dreyfus": true
"Jean-Paul Sartre": true
"Pat O'Brien": true
"Þór Eldon": false
"Marcus Wells-O'Shaugnessy": true
"Stephen Wells-O'Shaugnessy Marcus": true
"This-Is-A-Crazy-Name Jones": true
"---- --------": false
"'''' ''''''''": false
"'-'- -'-'-'-'": false
"a-'- b'-'-'-'": false
"'-'c -'-'-'-d": false
"e-'f g'-'-'-h": false
"'ij- -klmnop'": false

Note it still doesn't handle Unicode characters, but it could possibly be expanded to include those if needed.

If by word you mean the English letters a-z in upper or lower case, then:

/^(?:[a-z]+\\){2}[a-z]+$/i

That says:

• ^ Beginning of string
• (?:...) Non-capturing group
• [a-z]+ One or more letters a-z (or A-Z because of the i flag at the end). If you also want to allow some other characters, just add them to the [a-z] after the z. If you want to allow hyphens, add \- to it (you need the backslash, depending on where you put the hyphen, so I just always include it). Note that this is very English-centric, and even in English sometimes people write borrowed words with their non-English letters, such as résumé.
• \\ Backslash
• {2} Repeated twice
• (Then another word)
• $ End of string

The issues with your expression are:

• [a-Z] Is invalid because the range is out of order (Z comes before a). If it were valid (or if you wrote [Z-a]), it would matches everything between Z and a, which isn't just a-z and A-Z
• \\/ Requires a backslash and then a slash
• | is an alternation (this or that)
• \s is whitespace

If you are building something for modern browsers, there is something very pleasurable in HTML5:

<input id="username" name="name" type="text" pattern="[a-zA-Z]{5,}" title="Minimum 5 letters" required />

Reference: HTML5 forms input types

Update 2017/01/17:

Old browsers are quite weak on the market shares nowadays. This is a good practice to use HTML5 features instead of compatibility scripts.

Here's the validation function I came up with, tailor it for your own use-cases:

function isUserNameValid(username) {
  /* 
    Usernames can only have: 
    - Lowercase Letters (a-z) 
    - Numbers (0-9)
    - Dots (.)
    - Underscores (_)
  */
  const res = /^[a-z0-9_\.]+$/.exec(username);
  const valid = !!res;
  return valid;
}