# This is a file defining the BP grammar using BP syntax
#
# This is a complete definition of the grammar of BP grammar files, but it's
# mainly intended to be used as a proof-of-concept, and a stress-test for BP.
# The grammar files provided with BP are not otherwise intended to be full
# language grammars.

Grammar: __ *(Def [__`;])%__ __ ($$ / @!=(..$$%\n => "Could not parse this code"))
Def: @name=id __ `: __ (
      @definition=extended-pat
    / $$ @!=(''=>"No definition for rule")
    / @!=(..>(`;/id_`:/$)%\n => "Invalid definition: @0"))

# This is used for command line arguments:
String-pattern: ..$$ % (\n / Nodent / Escape / `\ pat [`;])

pat: simple-pat !(__("!="/"==")) / suffixed-pat
simple-pat: Upto-and / Dot / String / Chars / Nodent / Escape-range
    / Escape / Repeat / Optional / No / After / Before / Capture
    / Ref / parens

suffixed-pat: (
      Eq-pat
    / Not-eq-pat
)

Eq-pat: @first=pat__"=="__@second=pat
Not-eq-pat: @first=pat__"!="__@second=pat

Dot: `. !`.
String: (
        `" @s=*(Escape / !`".) (`" / @!=(''=> "Expected closing quote here"))
      / `' @s=*(Escape / !`'.) (`' / @!=(''=> "Expected closing quote here"))
    )
Chars: `` @+(Char-range/Char) % `,
Char-range: @low=. `- (@high=. / @!=(''=> "Expected a second character to form a character range"))
Char: (@s=. / @!=(''=> "Expected a character following the '`'"))
Escape-range: `\ @low=escape-sequence `- @high=escape-sequence
Escape: `\ (@s=escape-sequence
    / $ @!=(''=>"Backslashes are used for escape sequences, not splitting lines")
    / @!=(. *(Abc/`0-9) => "Invalid escape sequence: '@0'")
)
escape-sequence: (
       `n,t,r,e,b,a,v
      / 1-3 `0-7
      / `x 2 `0-9,a-f,A-F
    )
No: `! (__@pat / @!=(''=>"Expected a pattern after the exclamation mark"))
Nodent: `\ `N
Upto-and: ".." [__@first=simple-pat] [__`%__@second=simple-pat]
Repeat: (
        @min=(''=>'0') (`*=>"-") @max=(''=>'∞')
      / @min=int __ `- __ @max=int
      / @min=(int / ''=>'1') __ (`+=>"-") @max=(''=>'∞')
      / @min=@max=int
    ) __ @repeat-pat=pat [__`%__@sep=pat]
Optional: `[ __ extended-pat (__`] / @!=(''=> "Expected closing square bracket here"))
After: `< __ pat
Before: `> __ pat
Capture: `@ [__ @capture-name=(id/`!) __ !"=>" `=] __ (@capture=pat / @!=(''=> "Expected pattern to capture"))
Replace: (
      @replace-pat=(Replace / Chain / pat) __ "=>" (__ @replacement=String / @!=(''=> "Expected replacement string"))
    )
Ref: @name=id !(__`:)

parens: `( __ extended-pat (__ `) / @!=(''=> "Expected closing parenthesis here"))

Chain: 2+@(pat !(__"=>") / Replace)%__
Otherwise: 2+@(Replace / Chain / pat)%(__`/__)
extended-pat: Otherwise / Replace / Chain / pat

# Special-symbol rules:
_:  *(`  / \t)
__: *(`  / \t / \r / \n / comment)
$$: !(./\n)
$:  !.
^^: !<(./\n)
^:  !<.

id: "^^" / "^" / "__" / "_" / "$$" / "$" / "|" / `a-z,A-Z *`a-z,A-Z,0-9,-

comment: `# .. $