Could you please explain me the syntax in the following example or post me a link where is more general careful explanation of individual symbols? The help in vim is incomplete
:set foldexpr=getline(v:lnum)=~'^\\s*$'&&getline(v:lnum+1)=~'\\S'?'<1':1
what is unclear?
- Strings are between single quotes instead of double quotes? Or I can choose?
- What means the explanation mark (?) ? in
=~'\\S'?'<1':1 - What means the expression
'string1'string2'string3? - What means
:1?
Thank you
The foldexpr option supposed to contain an expression that evaluates
into an integer number or string of particular format that specifies the
folding level of the line which number is stored in the v:lnum global
variable at the moment of evaluation.
Let us follow the logic of this foldexpr example from top to bottom.
getline(v:lnum)=~'^\\s*$'&&getline(v:lnum+1)=~'\\S'?'<1':1
At the top level, the whole expression is a ternary operator A ? B : C.
The result of the operator is the value of the B expression if A
evaluates to non-zero, and the value of the C expression otherwise
(see :help expr1). In this case, B is the string literal
'<1', and C is the number 1 (for meaning of '<1' and 1 as fold
level specifiers see :help fold-expr).
The A expression consists of two conditions joined by the &&
operator:
getline(v:lnum) =~ '^\\s*$' && getline(v:lnum+1) =~ '\\S'
Both conditions have the same form
getline(N) =~ S
The getline function returns contents of the line (in the current
buffer) that is referenced by the line number passed as an argument (see
:help getline). When the foldexpr is evaluated, the v:lnum
variable contains number of the line for which folding level should be
calculated.
The =~ operator tests whether its left operand matches a regular
expression given by its right string operand, and returns boolean value
(see :help expr4, in particular, near the end of the expr4 section).
Thus, the A condition is intended to check that the v:lnum-th line
matches the '^\\s*$' pattern, and the line next to v:lnum-th one
matches the '\\S' pattern.
The regular expression patterns are specified in the expression as
string literals. String literals have two syntactic forms and can be
quoted using double or single quotes. The difference between these
forms is that double quoted string could contain various control
sequences which start with backslash. That sequences allow to specify
special characters that cannot be easily typed otherwise (double quote,
for example—it writes \"). Single quoted strings, at the other hand,
do not allow such backslash-sequences. (For complete description of
single and double quoted strings see :help expr-string and :help
literal-string.)
The notable consequence of the double quoted strings syntax is that
backslash symbol itself must be escaped (\\). That is why single
quoted strings are often used to specify regular expressions: there is
no need to escape constantly demanded backslash symbol. One can notice,
though, that backslashes are nevertheless escaped in those patterns
above. This is due to that some symbols (including backslash) have
special meaning when in Ex commands (including :set, of course). When
you hit Enter to start the command
:set foldexpr=...
Vim interprets some character sequences first (see :help
cmdline-special). In particular, the \\ sequence is treated as
a single backslash.
Putting it all together, the expression tests whether the line number
v:lnum contains only blank characters while the line next to it
(number v:lnum+1) has any non-blank character (see :help pattern to
grasp the meaning of the patterns). If so, the expression evaluates to
the string '<1', otherwise it evaluates to the number 1.
来源：https://stackoverflow.com/questions/5977895/explaining-foldexpr-syntax