snippet features and grammar, #3210

src/vs/editor/contrib/snippet/common/snippet.md

+
+Tabstops
+--
+
+With tabstops you can make the editor cursor move inside a snippet. Use `$1`, `$2` to specify cursor locations. The number is the order in which tabstops will be visited, whereas `$0` denotes the final cursor position. Multiple tabstops are linked and updated in sync.
+
+Placeholders
+--
+
+Placeholders are tabstops with values, like `${1:foo}`. The placeholder text will be inserted and selected such that it can be easily changed. Placeholders can nested, like `${1:another ${2:placeholder}}`.
+
+Variables
+--
+
+With `$name` or `${name:default}` you can insert the value of a variable. When a variable isn’t set its *default* or the empty string is inserted.
+
+
+Grammar
+--
+
+Below is the EBNF for snippets. The `$`-character can be escaped  using backslash, like `\$`.
+
+```
+any         ::= tabstop | placeholder | variable | text
+tabstop     ::= '$' int
+placeholder ::= '${' int '}' | '${' int ':' any '}'
+variable    ::= '$' var | '${' var }' | '${' var ':' any '}'
+var         ::= [_a-zA-Z] [_a-zA-Z0-9]*
+int         ::= [0-9]+
+text        ::= .*
+```