RegExp space character

Background

There are a couple of things to note here. First, a quantifier (in this case, {3,16}) only applies to the last regex token. So what your current regex really is saying is to "Match any string that has a single alphabetical character (case-insensitive) followed by 3 to 16 whitespace characters (e.g. spaces, tabs, etc.)."

Second, a name can have more than 2 parts (a middle name, certain ethnic names like "De La Cruz") or include special characters such as accented vowels. You should consider if this is something you need to account for in your program. These things are important and should be considered for any real application.

Assumptions and Answer

Now, let's just assume you only want a certain format for names that consists of a first name, a last name, and a space. Let's also assume you only want simple ASCII characters (i.e. no special characters or accented characters). Furthermore, both the first and last names should start with a capital character followed by only lower-case characters. Other than that, there are no restrictions on the length of the individual parts of the name. In this case, the following regex would do the trick:

^(?=.{3,16}$)[A-Z][a-z]+ [A-Z][a-z]+$

Notes

The first token after the ^ character is what is called a positive lookahead. Basically a positive look ahead will match the regex between the opening (?= and closing ) without actually moving the position of the cursor that is matching the string.

Notice I removed the \s token, since you usually want only a (space). The space can be replaced with the \s token, if tabs and other whitespace is desired there.

I also added a restriction that a name must start with a capital letter followed by only lower-case letters.

Crude English Translation

To help your understanding, here is a simple English translation of what the regex is really doing. The part in italics is just copied from the first part of the English translation of the regex.

"Match any string that has 3-16 characters and starts with a capital alphabetical character followed by one or more (+) alphabetical characters followed by a single space followed by a capital alphabetical character followed by one or more (+) alphabetical characters and ends with any lowercase letter."

Tools

There are a couple of tools I like to use when I am trying to tackle a challenging regex. They are listed below in no particular order:

• https://regex101.com/ - Allows you to test regex expressions in real time. It also has a nifty little library to help you along.
• http://www.regular-expressions.info/ - Basically a repository of knowledge on regex.

Edit/Update

You mentioned in your comments that you are using your regex in JavaScript. JavaScript uses a forward slash surrounding the regex to determine what is a regex. For this simple case, there are 2 options for using a regex to match a string.

First, use String's match method as follows

"John Smith".match(/^(?=.{3,16}$)[A-Z][a-z]+ [A-Z][a-z]+$/);

Second, create a regex and use its test() method. For example,

/^(?=.{3,16}$)[A-Z][a-z]+ [A-Z][a-z]+$/.test("John Smith");

The latter is probably what you want as it simply returns true or false depending on whether the regex actually matches the string or not.