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While there's normally no problem with having the pound sign # in an \href link (see Hyperlink with # (number sign)), it does not work within a frame environment in beamer.

\documentclass{beamer} \usepackage{hyperref} \begin{document} \begin{frame} \href{http://de.wikipedia.org/wiki/Hybridorbital#Beispiele}{Wiki} \end{frame} \end{document}

This results in the error ! Illegal parameter number in definition of \iterate.

Remove the frame environment, though, and it compiles fine. So my question is, how can I include an \href link with the symbol # in a frame environment in beamer?

EDIT: A comment suggests to add the option [fragile] to the frame environment. While that takes care of the MWE above, it won't fix the fact the MWE below also crashes because of the pound sign # in the \href link:

\documentclass{beamer} \usepackage{hyperref} \begin{document} \begin{frame}[fragile] \only<1>{\href{http://de.wikipedia.org/wiki/Hybridorbital#Beispiele}}{Wiki} \end{frame} \end{document}

This gives the error ! Illegal parameter number in definition of \beamer@doifinframe. Remove the pound sign # from the link, though, and it compiles without error.

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    Frames with verbatim content (\href is sort of verbatim) require the fragile option: \begin{frame}[fragile]. Commented Aug 16, 2019 at 12:36
  • @PhelypeOleinik Add \only<1>{} to the frame content, and it breaks again, even with [fragile]. Commented Aug 16, 2019 at 12:48
  • Only if you do \only<1>{\href{...}}. As with all verbatim commands, \href usually won't work as argument to another command. Commented Aug 16, 2019 at 12:51
  • @PhelypeOleinik It works without the pound sign #, so it's not the \href itself which is the problem here. It's its combination with #. Commented Aug 16, 2019 at 12:53
  • without the # it works as \href doesn't have to handle a special char, as soon as you use special chars you can't use \href in the argument of another command. Use an environment instead of \only. Commented Aug 16, 2019 at 13:00

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beamer scans the entire frame environment before actually typesetting anything, and this leads to problems with verbatim content, because it cannot be "seen" before it is actually being typeset, or the tokenization will happen too early and it will not work.

Essentially, what happens in your first example is:

\def\iterate{Hybridorbital#Beispiele}

which throws the error:

! Illegal parameter number in definition of \iterate. <to be read again> B l.4 \def\iterate{Hybridorbital#B

The next example shows the same issue, except that the problem doesn't happen while beamer is scanning the frame, but when it grabs the argument of \only. As a rule of thumb, con't use verbatim commands as argument to other commands.

To work around the issue, you can escape the #. The documentation of \href says:

enter image description here

So this works (because \# is not the "special" character #):

\documentclass{beamer} \usepackage{hyperref} \begin{document} \begin{frame} \only<1>{\href{http://de.wikipedia.org/wiki/Hybridorbital\#Beispiele}{Wiki}} \end{frame} \end{document}
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    I thought the escape character \ was only for the text argument, not the url argument, and that putting \ in the url would be interpreted literally. Good to know! Commented Aug 16, 2019 at 13:11
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    @Sverre TeX in general is tricky: everything can have a different meaning in a different context. \href makes sure that both # and \# mean the character #. Commented Aug 16, 2019 at 13:20
  • When I do this, XeLaTeX converts the \# to %23 in the URL, spoiling the hyperlink. Commented Oct 15, 2021 at 20:34
  • @jvriesem That's not XeLaTeX or hyperref, it's (very, very likely) your PDF viewer: github.com/latex3/hyperref/issues/110. Are you by chance using Preview, Skip, Safari or TeXShop? Commented Oct 16, 2021 at 2:06

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