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I want to write a snippet for LaTeX sum that behaves like this: I write "sum" and trigger snippet expansion. "sum" becomes "\sum_{$1}^{$2} $0", with $1 being the first entry point, $2 the second one and $0 the exit point.

If I write nothing inside $1, I want to have the "_{}" part deleted and proceed to $2. Likewise with $2.

Is such a thing possible with YASnippet or any other snippet engine? I tried putting an Elisp function that deletes the needed part if exiting with no input made but it breaks the snippet.

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You can do this with an extra step (see "Nested placeholder fields" in the manual):

\sum${1:_{${2:}}}^{$3} $0

With the optional field: TAB, TAB, insert field 1, TAB, insert field 2, TAB to exit. Without the optional field: TAB, C-d, insert field 2, TAB to exit.

Another option is to use a function that deletes the empty field in case it finds one:

\sum_{$1}^{$2} ${0:$$(yas-delete-if-empty)} (defun yas-delete-if-empty () (save-excursion (when (re-search-backward "\\\\sum\\(_{}\\)^{.+}" (line-beginning-position) t) (replace-match "" t t nil 1))))
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