Meaning of '+' at the end of pattern in HTML [duplicate]

In HTML when using patter in input, it works when in the end it has '+', and doesnt work when there is no plus sign. Example:

<input name = "username" patterm = "[^' ']+">
<input name = "username" patterm = "[^' ']">

these lines should only allow strings without space. The first one does exactly that, but the second one always says that string doesn't meet requirements. Why is that plus sign necessary at the end?