The expression used for when 'foldmethod' is "expr". It is evaluated for each line to obtain its fold level. The context is set to the script where 'foldexpr' was set, script-local items can be accessed. See fold-expr for the usage.

The expression will be evaluated in the sandbox if set from a modeline, see sandbox-option. This option can't be set from a modeline when the 'diff' option is on or the 'modelineexpr' option is off.

It is not allowed to change text or jump to another window while evaluating 'foldexpr' textlock.

(default: "0")

not available when compiled without the +folding or +eval features