support syntax highlighting for bracketed expression in fmt string

[e-zhang]

Per the go docs for the fmt package, there is a bracket expression that is supported in format strings.

For example,
fmt.Sprintf("%[2]d %[1]d\n", 11, 22) fmt.Sprintf("%[3]*.[2]*[1]f", 12.0, 2, 6) fmt.Sprintf("%d %d %#[1]x %#x", 16, 17)

Should all be valid syntax highlighted format strings.

[arp242]

Thanks! Seems to work well judging from looking at fmt/fmt_test.go in Vim. I added a commit to support %F too, as that didn't work either.

The only pattersn that don't work yet are:

{"%.f", 0i, "(0+0i)"},
{"% .f", 0i, "( 0+0i)"},
{"%+.f", 0i, "(+0+0i)"},
{"% +.f", 0i, "(+0+0i)"},

I tried to fix it, but leaning toothpick syndrome hurts by brain at this hour. I think we can make this more readable by some strategic insertions of \v and such.

I also wonder if there's a good way to split this across multiple lines, preferably with comments like in Python or Ruby:

 pattern = """
    ^                   # beginning of string
    M{0,4}              # thousands - 0 to 4 M's
    (CM|CD|D?C{0,3})    # hundreds - 900 (CM), 400 (CD), 0-300 (0 to 3 C's),
                        #            or 500-800 (D, followed by 0 to 3 C's)
    (XC|XL|L?X{0,3})    # tens - 90 (XC), 40 (XL), 0-30 (0 to 3 X's),
                        #        or 50-80 (L, followed by 0 to 3 X's)
    (IX|IV|V?I{0,3})    # ones - 9 (IX), 4 (IV), 0-3 (0 to 3 I's),
                        #        or 5-8 (V, followed by 0 to 3 I's)
    $                   # end of string
    """

That would make it much easier to maintain!

[e-zhang]

I will take a look at those and learn to run the units as well. Thanks for the feedback, I'll try to update this PR with those changes as soon as a I can.

[arp242]

Here's the best I could come up with:

diff --git i/syntax/go.vim w/syntax/go.vim
index 89b0e9a..a7bc2e1 100644
--- i/syntax/go.vim
+++ w/syntax/go.vim
@@ -202,7 +202,19 @@ else
 endif
 
 if g:go_highlight_format_strings != 0
-  syn match       goFormatSpecifier   /\([^%]\(%%\)*\)\@<=%[-#0 +]*\%(\%(\%(\[\d\]\)\=\*\)\|\d\+\)\=\%(\.\%(\%(\%(\[\d\]\)\=\*\)\|\d\+\)\)\=\%(\[\d\]\)\=[vTtbcdoqxXUeEfFgGsp]/ contained containedin=goString,goRawString
+  " 1. Match a literal % not preceded by a %.
+  " 2. Match -, #, 0, space, or +
+  " 3. ??
+  " 4. ??
+  " 5. ??
+  " 6. Match formatting verb.
+  syn match       goFormatSpecifier   /\
+                  \([^%]\(%%\)*\)\
+                  \@<=%[-#0 +]*\
+                  \%(\%(\%(\[\d\]\)\=\*\)\|\d\+\)\=\
+                  \%(\.\%(\%(\%(\[\d\]\)\=\*\)\|\d\+\)\)\=\
+                  \%(\[\d\]\)\
+                  \=[vTtbcdoqxXUeEfFgGsp]/ contained containedin=goString,goRawString
   hi def link     goFormatSpecifier   goSpecialString
 endif

Vim is very finicky about where you can place the \ though, and it doesn't always work :-(

Maybe a better solution is to use string concatenation (untested):

" Match a literal % not preceded by a %.
let s:p = '\([^%]\(%%\)*\)\'

" Match -, #, 0, space, or +
let s:p .= '\@<=%[-#0 +]*\'

[...]

exe 'syn match goFormatSpecifier /' . s:p . '/ contained containedin=goString,goRawString'

[e-zhang]

Took your suggestion for doing multi-line continuation and added comments. I think everything in fmt/fmt_test.go is highlighted correctly now.

I'm not familiar enough with the \v option in vim, so I punted on doing that for now.

[arp242]

Thanks! Works well, and much more readable/maintainable like this, even though it's still a long/ugly regex :-)

[fatih]

Nice addition 👍