highlight-regexp is not part of library Highlight (
highlight.el). It's part of vanilla Emacs. Functions and variables defined by library
highlight.el use prefix
hlt-. Only library
highlight.el lets you use
highlight.el command for highlighting regexp matches in the region (or the buffer) is
hlt-highlight-regexp-region. Try that, instead of
Be aware too that vanilla command
highlight-regexp works with
hi-lock-mode, which means it works (only) with
font-lock-mode turned on.
Features of library
highlight.el can be used to highlight text whether or not font locking is on. To understand how to do this, see Interaction with Font Lock for using
highlight.el with and without
font-lock-mode being on.