How can I reject files named '!' with wget? - regex

Question

I'm using wget to recursively download my university's pages for later analysis and am filtering lots of extensions.
Here's a mwe with the relevant function:
#!/bin/sh
unwanted_extensions='*.apk,*.asc,*.asp,*.avi,*.bat,*.bib,*.bmp,*.bz2,*.c,*.cdf,*.cgi,*.class,*.cpp,*.crt,*.csp,*.css,*.cur,*.dat,*.dll,*.dvi,*.dwg,*.eot,*.eps,*.epub,*.exe,*.f,*.flv,*.for,*.ggb,*.gif,*.gpx,*.gz,*.h,*.heic,*.hpp,*.hqx,*.htc,*.ico,*.jfif,*.jpe,*.jpeg,*.jpg,*.js,*.lib,*.lnk,*.ly,*.m,*.m4a,*.m4v,*.mdb,*.mht,*.mid,*.mp3,*.mp4,*.mpeg,*.mpg,*.mso,*.odb,*.ogv,*.otf,*.out,*.pdb,*.pdf,*.php,*.plot,*.png,*.ps,*.psz,*.py,*.rar,*.sav,*.sf3,*.sgp,*.sh,*.sib,*.svg,*.swf,*.tex,*.tgz,*.tif,*.tiff,*.tmp,*.ttf,*.txt,*.wav,*.webm,*.webmanifest,*.webp,*.wmf,*.woff,*.woff2,*.wxm,*.wxmx,*.xbm,*.xml,*.xps,*.zip'
unwanted_regex='/([a-zA-Z0-9]+)$'
wget_custom ()
{
link="$1"
wget \
--recursive -e robots=off --level=inf --quiet \
--ignore-case --adjust-extension --convert-file-only \
--reject "$unwanted_extensions" \
--reject-regex "$unwanted_regex" --regex-type posix \
"$link"
}
wget_custom "$1"
It works nicely and filters most of the stuff. However, these webs serve many pdf and image files named ! (e.g. biologiacelular.ugr.es/pages/planoweb/!) which I don't need and want to reject. Here's what i've tried but hasn't worked:
Appending ,! to unwanted_extensions
Appending ,%21 to unwanted_extensions
Changing unwanted_regex to '/([a-zA-Z0-9!]+)$'
Changing unwanted_regex to '/([a-zA-Z0-9\!]+)$'
Adding nother --reject-regex '/!$
Adding nother --reject-regex '/\!$
None of these work and I'm out of ideas. How can I filter the ! files? Thank you!