I need help with regular expression.I need a expression in jquery which allows only character or space between two words. no double space allowed

I am using this

/^[a-zA-Z]+(-_ [a-zA-Z]+)*/

but it's not working.

Example
space hello - not allowed

hello space - not allowed

hello space space hello - not allowed

space hello space - not allowed

hello1234 - not allowed

hello space 1234 - not allowed

hello world-allowed

hello-allowed

hello how are you-allowed