什么是巴科斯范式？

什么是巴科斯范式？



巴科斯范式(BNF: Backus-Naur Form 的缩写)是由 John Backus 和 Peter Naur 首先引入的用来描述计算机语言语法的符号集。

现在，几乎每一位新编程语言书籍的作者都使用巴科斯范式来定义编程语言的语法规则。



巴科斯范式的内容



在双引号中的字("word")代表着这些字符本身。而double_quote用来代表双引号。

在双引号外的字（有可能有下划线）代表着语法部分。

尖括号( < > )内包含的为必选项。

方括号( [ ] )内包含的为可选项。

大括号( { } )内包含的为可重复0至无数次的项。

竖线( | )表示在其左右两边任选一项，相当于"OR"的意思。

::= 是“被定义为”的意思。



巴科斯范式示例



这是用BNF来定义的Java语言中的For语句的实例：



FOR_STATEMENT ::=

"for" "(" ( variable_declaration |

( expression ";" ) | ";" )

[ expression ] ";"

[ expression ] ";"

")" statement



这是Oracle packages的BNF定义：



package_body ::= "package" package_name "is"

package_obj_body { package_obj_body }

[ "begin" seq_of_statements ]

"end" [ package_name ] ";"



package_obj_body ::= variable_declaration

| subtype_declaration

| cursor_declaration

| cursor_body

| exception_declaration

| record_declaration

| plsql_table_declaration

| procedure_body

| function_body



procedure_body ::= "procedure" procedure_name

[ "(" argument { "," argument } ")" ]

"return" return_type

"is"

[ "declare" declare_spec ";" { declare_spec ";" } ]

"begin"

seq_of_statements

[ "exception" exception_handler { exception_handler } ]

"end" [ procedure_name ] ";"



statement ::= comment

| assignment_statement

| exit_statement

| goto_statement

| if_statement

| loop_statement

| null_statement

| raise_statement

| return_statement

| sql_statement

| plsql_block





这是用BNF来定义的BNF本身的例子：



syntax ::= { rule }

rule ::= identifier "::=" expression

expression ::= term { "|" term }

term ::= factor { factor }

factor ::= identifier |

quoted_symbol |

"(" expression ")" |

"[" expression "]" |

"{" expression "}"

identifier ::= letter { letter | digit }

quoted_symbol ::= """ { any_character } """