One way to ensure reliable rehighlighting of multiline Font Lock
constructs is to put on them the text property
It should be present and non-
nil for text that is part of a
When Font Lock is about to highlight a range of text, it first
extends the boundaries of the range as necessary so that they do not
fall within text marked with the
Then it removes any
font-lock-multiline properties from the
range, and highlights it. The highlighting specification (mostly
font-lock-keywords) must reinstall this property each time,
whenever it is appropriate.
Warning: don't use the
on large ranges of text, because that will make rehighlighting slow.
font-lock-multilinevariable is set to
t, Font Lock will try to add the
font-lock-multilineproperty automatically on multiline constructs. This is not a universal solution, however, since it slows down Font Lock somewhat. It can miss some multiline constructs, or make the property larger or smaller than necessary.
For elements whose matcher is a function, the function should ensure that submatch 0 covers the whole relevant multiline construct, even if only a small subpart will be highlighted. It is often just as easy to add the
font-lock-multilineproperty by hand.
font-lock-multiline property is meant to ensure proper
refontification; it does not automatically identify new multiline
constructs. Identifying them requires that Font Lock mode operate on
large enough chunks at a time. This will happen by accident on many
cases, which may give the impression that multiline constructs magically
work. If you set the
nil, this impression will be even stronger, since the
highlighting of those constructs which are found will be properly
updated from then on. But that does not work reliably.
To find multiline constructs reliably, you must either manually place
font-lock-multiline property on the text before Font Lock
mode looks at it, or use