一、词法文件结构

flex的词法输入文件.l 结构通过%%分成三部分

• 声明

• 规则

• c 代码

声明中 %{ C代码声明 %} 中包裹的内容将直接原样拷贝到生成的C代码中，

最后的段也是直接原样拷贝到生成的C代码中，

中间的规则段，每个规则由 模式 + 动作 组成，模式一般都是正则表达式书写方式。

二、pg中的词法分析文件

2.1 声明段

src/backend/parser/scan.l

%{

/* LCOV_EXCL_START */

/* Avoid exit() on fatal scanner errors (a bit ugly -- see yy_fatal_error) */
#undef fprintf
#define fprintf(file, fmt, msg)  fprintf_to_ereport(fmt, msg)

static void
fprintf_to_ereport(const char *fmt, const char *msg)
{
	ereport(ERROR, (errmsg_internal("%s", msg)));
}

/*
 * GUC variables.  This is a DIRECT violation of the warning given at the
 * head of gram.y, ie flex/bison code must not depend on any GUC variables;
 * as such, changing their values can induce very unintuitive behavior.
 * But we shall have to live with it until we can remove these variables.
 */
int			backslash_quote = BACKSLASH_QUOTE_SAFE_ENCODING;
bool		escape_string_warning = true;
bool		standard_conforming_strings = true;
...

%}

...
%option nodefault
%option noinput
%option nounput
%option noyywrap
%option noyyalloc
%option noyyrealloc
%option noyyfree
%option warn
%option prefix="core_yy"

...

%%

2.2 规则段

{whitespace}	{
					/* ignore */
				}

{xcstart}		{
					/* Set location in case of syntax error in comment */
					SET_YYLLOC();
					yyextra->xcdepth = 0;
					BEGIN(xc);
					/* Put back any characters past slash-star; see above */
					yyless(2);
				}

<xc>{
{xcstart}		{
					(yyextra->xcdepth)++;
					/* Put back any characters past slash-star; see above */
					yyless(2);
				}
...

%%

/* LCOV_EXCL_STOP */

2.3 C code 段

/*
 * Arrange access to yyextra for subroutines of the main yylex() function.
 * We expect each subroutine to have a yyscanner parameter.  Rather than
 * use the yyget_xxx functions, which might or might not get inlined by the
 * compiler, we cheat just a bit and cast yyscanner to the right type.
 */
#undef yyextra
#define yyextra  (((struct yyguts_t *) yyscanner)->yyextra_r)

/* Likewise for a couple of other things we need. */
#undef yylloc
#define yylloc	(((struct yyguts_t *) yyscanner)->yylloc_r)
#undef yyleng
#define yyleng	(((struct yyguts_t *) yyscanner)->yyleng_r)


/*
 * scanner_errposition
 *		Report a lexer or grammar error cursor position, if possible.
 *
 * This is expected to be used within an ereport() call, or via an error
 * callback such as setup_scanner_errposition_callback().  The return value
 * is a dummy (always 0, in fact).
 *
 * Note that this can only be used for messages emitted during raw parsing
 * (essentially, scan.l, parser.c, and gram.y), since it requires the
 * yyscanner struct to still be available.
 */
int
scanner_errposition(int location, core_yyscan_t yyscanner)
{
	int			pos;

	if (location < 0)
		return 0;				/* no-op if location is unknown */

	/* Convert byte offset to character number */
	pos = pg_mbstrlen_with_len(yyextra->scanbuf, location) + 1;
	/* And pass it to the ereport mechanism */
	return errposition(pos);
}

...

2.4 最终生成的词法分析文件

src/backend/parser/scan.c
通过flex工具，对scan.l文件进行处理，最终将生成scan.c代码，用于后续对sql语言进行词法分析。