xparse command with just optional argument fails

Question

I am trying to define a command with xparse that takes an optional argument but no mandatory argument. The optional argument is a number, which works fine with a single digit but not with more. The command is as follows:

\usepackage{xparse}
\usepackage{xspace}

\NewDocumentCommand \HL { o } {\IfNoValueTF #1 {\emph{HL}\xspace}{\emph{HL} {#1}\xspace}}

\HL
(\HL[1])
(\HL[10])

I want the following result:

HL

(HL 1)

(HL 10)

But I get this:

HL

(HL 1)

(HLHL 10)

Answer 1

As #1 is potentially more than one token you should always handle it in braces. Thus you need

\IfNoValueTF {#1}

which prevents the second or later tokens of #1 being misunderstood as one the the true/false branches. In your case, \HL[10] is only testing 1 and treating the 0 as the true branch for the conditional, leading to the odd output.