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*28.8* Folding by expression

This is similar to folding by indent, but instead of using the indent of a
line a user function is called to compute the fold level of a line.  You can
use this for text where something in the text indicates which lines belong
together.  An example is an e-mail message where the quoted text is indicated
by a ">" before the line.  To fold these quotes use this: >

:set foldmethod=expr
:set foldexpr=strlen(substitute(substitute(getline(v:lnum),'\\s','',\"g\"),'[^>].*','',''))

You can try it out on this text:

> quoted text he wrote
> quoted text he wrote
> > double quoted text I wrote
> > double quoted text I wrote

Explanation for the 'foldexpr' used in the example (inside out):
   getline(v:lnum) gets the current line
   substitute(...,'\\s','','g') removes all white space from the line
   substitute(...,'[^>].*','','') removes everything after leading '>'s
   strlen(...) counts the length of the string, which
is the number of '>'s found

Note that a backslash must be inserted before every space, double quote and
backslash for the ":set" command.  If this confuses you, do >

:set foldexpr

to check the actual resulting value.  To correct a complicated expression, use
the command-line completion: >

:set foldexpr=<Tab>

Where <Tab> is a real Tab.  Vim will fill in the previous value, which you can
then edit.

When the expression gets more complicated you should put it in a function and
set 'foldexpr' to call that function.

More about folding by expression in the reference manual: |fold-expr|

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