Regex allowing a space character in Java

Hey all, so I'm trying to allow some text input which goes through a regex check before it's sent off. I want the text to only include A-Z, 0-9, and the space " " character. Here is my code now:

if(!title.matches("[a-zA-Z0-9_]+") {
    //fail
}
else {
    //success
}

However this still gives the //fail result when I input something like "This is a test"

Any ideas? Thanks all.