How to restrict more than one whitespace in Java Regex?

Question

A validation to be developed for a JavaFX text field where single whitespace is allowed but more than one whitespace is not be allowed.

For example, "Apple Juice" -- correct "Apple Juice" -- incorrect should be restricted

if (title.matches("[a-zA-Z0-9 ]+"))

Found couple of links but not meeting my requirement. I believe that it is more of a logical tweak.

Whitespace Matching Regex - Java

Regex allowing a space character in Java

Answer 1

You can do:

if (title.matches([a-zA-z]+[ ][a-zA-Z]+))

• The first [a-zA-z]+ checks for any characters before the space.
• The [ ] checks for exactly one space.
• The second [a-zA-z]+ checks for any characters after the space.

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Note: This will match only if the space is present in between the string. If you want to match strings like Abcd<space> or <space>Abcd, (I used <spcace> as SO does not allow two spaces to be present simultaneously) then you can replace the +s with *s., i.e.,

if (title.matches([a-zA-z]*[ ][a-zA-Z]*))

Answer 2

You could do

if ("Apple Juice".matches("\\w+ \\w+")) {
 .......

Answer 3

You'd better find more than a single whitespace and negate the result:

if(!title.trim().matches("\s{2,}"))

, see java.util.regex.Pattern javadoc for the syntax. The string is first trimmed, so you don't need to check for non-whitespace characters. If you don't do the trim() operation, leading and trailing whitespace will also be considered.