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<!--#if expr="$variable = /foo¦bar¦sna/" -->
As written, it matches words like "food" & "snack".
I realize I could split it up into 3 non-regular expression conditional statements ($variable=foo ¦¦ $variable=bar, etc.), but I'd like to keep it together for elegance and maintainability.
When using a regex, then you need to anchor your pattern to the start and the end of the string:
<!--#if expr="$variable = /[b]^[/b](foo¦bar¦sna)[b]$[/b]/" -->
Alternatively, you could use the or operator, which is written as
<!--#if expr="$variable = foo [b]¦¦[/b] $variable = bar [b]¦¦[/b] $variable = sna" -->
I haven't actually tested either variation, but they should at least give you a start into the right direction.
For my application I'm going to use the following to get an exact match on "foo" and substring matches for the rest.
<!--#if expr="$QUERY_STRING = /^foo$¦bar¦sna/" -->
Thanks for getting me pointed in the right direction!