Syntax highlight rule for a block comment using indentation

Question

I have the following code and I want to define a syntax highlight rule that will show all lines as commented out starting at the line with ## till the indentation ends just before the last print line.

The indentation works like that in python.

function test a b
    arg Int a b
    ## if a>b
        show "Bigger"
        another command
    print "More stuff"

I think I should use something like the following, but how do I define the end condition based on indentation ? The start condition should somehow remember its indentation and the end condition should use that information. But how ?

syntax region myComment start=/\v##/ skip=/\v\\./ end=???

The comment highligthing should also stop when encountering a blank line.

function test a b
    arg Int a b
    ## if a>b
       show "Bigger"

function more a b
    arg Int a b

Answer 1

You can try following:

syntax region Comment start=/^\z(\s*\)##/ end=/^\z1\ze\S/

See :h /\z(\).

Basically it saves whitespace indentation in \z1 and use it in end of region just before first non-whitespace. Also see :h /\ze.

For Edited part of question

Make it also end on empty line

syntax region Comment start=/^\z(\s*\)##/ end=/\(^\z1\ze\S\)\|\(^\s*$\)/

by adding empty line regex to the end.

Bear in mind that it probably wouldn't work if you mix tabs and spaces.