Vim syntax region - lookbehind confusion

lookbehind, match, regex, syntax, vim

Solution

Updated: At `f`, looking behind, you probably want to check for `[. \t\n\r]`, not `[^. \t\n\r]`. Because currently, you're saying "something that doesn't follow one of these characters", so only upon reaching the `o` is that condition met, since `f` is indeed not one of those characters. So you have to either un-negate the character class, or as you discovered, negate the lookbehind.

I think you're getting your terms confused, too.

\@<=    positive lookbehind
\@<!    negative lookbehind
\@=     positive lookahead
\@!     negative lookahead

Problem

Define the following in .vimrc or execute within vim command line: ``` syn match ndbMethods "[^. \t\n\r]\@<=[_a-z][_a-zA-Z0-9]*(\@=" hi ndbMethods guibg=#222222 ``` View results with a C-style method call in the active buffer: ``` foo(); ``` You will see the initial character of the method name is not matched. The intention is for the lookbehind pattern to force a beginning of line, literal `.` or whitespace to precede any matched method's first character. Oddly enough, making this a negative lookahead (`\@<!`) seems to work! Would someone be kind enough to explain why this lookbehind is incorrect?

Original source