/(?(?{! whatever() })(?!))/ #### /(?(?{ whatever() })|(?!))/ #### /(?(?{ whatever() })|(*FAIL))/ #### /(?{ whatever() })/ #### /(?{ whatever() }?)/
## /(?(?{ whatever() })|(?!))/ ##
## /(?(?{ whatever() })|(*FAIL))/ ##
## /(?{ whatever() })/ ##
## /(?{ whatever() }?)/