本篇内容介绍了“PostgreSQL中的Rules有什么作用”的有关知识，在实际案例的操作过程中，不少人都会遇到这样的困境，接下来就让小编带领大家学习一下如何处理这些情况吧！希望大家仔细阅读，能够学有所成！
Flex输入文件由四部分组成:
%{ Declarations %} Definitions %% Rules %% User subroutines在Flex的模式文件中，%%和%%之间的内容被称为规则（rules），每一行表示一条规则，每条规则由匹配模式（pattern）和 动作（action）组成。其中模式在前面，用正则表达式表示，动作在后面，即C代码。每当一个模式被匹配到时，后面的C代码将被执行。
Flex会将规则翻译成名为yylex的函数，该函数扫描输入文件（默认标准输入），当扫描到一个完整的、最长的、可以和某条规则的正则表达式所匹配的输入时，函数会执行此规则后面的C代码。如果代码中没有return语句，则执行完毕后，yylex会继续运行，开始下一轮的扫描和匹配。注意：当有多条规则的模式被匹配到时， yylex会优先选择匹配长度最长的那条规则，如果有匹配长度相等的规则，则选择排在最前面的那条规则。
PG中的规则定义如下：
%% {whitespace} { //--------- 空白字符 //忽略,不作任何处理 /* ignore */ } {xcstart} { //--------- C风格注释 /* Set location in case of syntax error in comment */ //设置位置,以防注释中的语法错误 SET_YYLLOC(); //深度 yyextra->xcdepth = 0; //进入xc状态 BEGIN(xc); /* Put back any characters past slash-star; see above */ //把斜杠星后的字符放回去 // 注意:"/*"是2个字符,从位置2(偏移从0起算)开始把之后的字符放回去 yyless(2); } <xc>{xcstart} { //遇到下一层的注释,深度+1 (yyextra->xcdepth)++; /* Put back any characters past slash-star; see above */ //类似的,把之后的字符放回去 yyless(2); } <xc>{xcstop} { //层次≤0,回到INITIAL状态,否则层次减1 if (yyextra->xcdepth <= 0) BEGIN(INITIAL); else (yyextra->xcdepth)--; } <xc>{xcinside} { //注释里面的内容,忽略 /* ignore */ } <xc>{op_chars} { //注释里面的内容,忽略 /* ignore */ } <xc>\*+ { //注释里面的内容,忽略 /* ignore */ } <xc><<EOF>> { yyerror("unterminated /* comment"); }//遇到结束符,出错 {xbstart} { /* Binary bit type. * At some point we should simply pass the string * forward to the parser and label it there. * In the meantime, place a leading "b" on the string * to mark it for the input routine as a binary string. */ //--------- 二进制位串 //在某些点上,我们应该简单的把字符串向前传递给解析器并标记它 //在此期间,设置一个打头的字符"b"以标记该输入为二进制串 SET_YYLLOC(); BEGIN(xb); startlit(); addlitchar('b', yyscanner); } <xb>{quotestop} | <xb>{quotefail} { yyless(1); BEGIN(INITIAL); yylval->str = litbufdup(yyscanner); return BCONST; } <xh>{xhinside} | <xb>{xbinside} { addlit(yytext, yyleng, yyscanner); } <xh>{quotecontinue} | <xb>{quotecontinue} { /* ignore */ } <xb><<EOF>> { yyerror("unterminated bit string literal"); } {xhstart} { //------------- 十六进制串 /* Hexadecimal bit type. * At some point we should simply pass the string * forward to the parser and label it there. * In the meantime, place a leading "x" on the string * to mark it for the input routine as a hex string. */ SET_YYLLOC(); BEGIN(xh); startlit(); addlitchar('x', yyscanner); } <xh>{quotestop} | <xh>{quotefail} { yyless(1); BEGIN(INITIAL); yylval->str = litbufdup(yyscanner); return XCONST; } <xh><<EOF>> { yyerror("unterminated hexadecimal string literal"); } {xnstart} { //------------- 国家字符 /* National character. * We will pass this along as a normal character string, * but preceded with an internally-generated "NCHAR". */ const ScanKeyword *keyword; SET_YYLLOC(); yyless(1); /* eat only 'n' this time */ keyword = ScanKeywordLookup("nchar", yyextra->keywords, yyextra->num_keywords); if (keyword != NULL) { yylval->keyword = keyword->name; return keyword->value; } else { /* If NCHAR isn't a keyword, just return "n" */ yylval->str = pstrdup("n"); return IDENT; } } {xqstart} { yyextra->warn_on_first_escape = true; yyextra->saw_non_ascii = false; SET_YYLLOC(); if (yyextra->standard_conforming_strings) BEGIN(xq); else BEGIN(xe); startlit(); } {xestart} { yyextra->warn_on_first_escape = false; yyextra->saw_non_ascii = false; SET_YYLLOC(); BEGIN(xe); startlit(); } {xusstart} { SET_YYLLOC(); if (!yyextra->standard_conforming_strings) ereport(ERROR, (errcode(ERRCODE_FEATURE_NOT_SUPPORTED), errmsg("unsafe use of string constant with Unicode escapes"), errdetail("String constants with Unicode escapes cannot be used when standard_conforming_strings is off."), lexer_errposition())); BEGIN(xus); startlit(); } <xq,xe>{quotestop} | <xq,xe>{quotefail} { yyless(1); BEGIN(INITIAL); /* * check that the data remains valid if it might have been * made invalid by unescaping any chars. */ if (yyextra->saw_non_ascii) pg_verifymbstr(yyextra->literalbuf, yyextra->literallen, false); yylval->str = litbufdup(yyscanner); return SCONST; } <xus>{quotestop} | <xus>{quotefail} { /* throw back all but the quote */ yyless(1); /* xusend state looks for possible UESCAPE */ BEGIN(xusend); } <xusend>{whitespace} { /* stay in xusend state over whitespace */ } <xusend><<EOF>> | <xusend>{other} | <xusend>{xustop1} { /* no UESCAPE after the quote, throw back everything */ yyless(0); BEGIN(INITIAL); yylval->str = litbuf_udeescape('\\', yyscanner); return SCONST; } <xusend>{xustop2} { /* found UESCAPE after the end quote */ BEGIN(INITIAL); if (!check_uescapechar(yytext[yyleng - 2])) { SET_YYLLOC(); ADVANCE_YYLLOC(yyleng - 2); yyerror("invalid Unicode escape character"); } yylval->str = litbuf_udeescape(yytext[yyleng - 2], yyscanner); return SCONST; } <xq,xe,xus>{xqdouble} { addlitchar('\'', yyscanner); } <xq,xus>{xqinside} { addlit(yytext, yyleng, yyscanner); } <xe>{xeinside} { addlit(yytext, yyleng, yyscanner); } <xe>{xeunicode} { pg_wchar c = strtoul(yytext + 2, NULL, 16); check_escape_warning(yyscanner); if (is_utf16_surrogate_first(c)) { yyextra->utf16_first_part = c; BEGIN(xeu); } else if (is_utf16_surrogate_second(c)) yyerror("invalid Unicode surrogate pair"); else addunicode(c, yyscanner); } <xeu>{xeunicode} { pg_wchar c = strtoul(yytext + 2, NULL, 16); if (!is_utf16_surrogate_second(c)) yyerror("invalid Unicode surrogate pair"); c = surrogate_pair_to_codepoint(yyextra->utf16_first_part, c); addunicode(c, yyscanner); BEGIN(xe); } <xeu>. { yyerror("invalid Unicode surrogate pair"); } <xeu>\n { yyerror("invalid Unicode surrogate pair"); } <xeu><<EOF>> { yyerror("invalid Unicode surrogate pair"); } <xe,xeu>{xeunicodefail} { ereport(ERROR, (errcode(ERRCODE_INVALID_ESCAPE_SEQUENCE), errmsg("invalid Unicode escape"), errhint("Unicode escapes must be \\uXXXX or \\UXXXXXXXX."), lexer_errposition())); } <xe>{xeescape} { if (yytext[1] == '\'') { if (yyextra->backslash_quote == BACKSLASH_QUOTE_OFF || (yyextra->backslash_quote == BACKSLASH_QUOTE_SAFE_ENCODING && PG_ENCODING_IS_CLIENT_ONLY(pg_get_client_encoding()))) ereport(ERROR, (errcode(ERRCODE_NONSTANDARD_USE_OF_ESCAPE_CHARACTER), errmsg("unsafe use of \\' in a string literal"), errhint("Use '' to write quotes in strings. \\' is insecure in client-only encodings."), lexer_errposition())); } check_string_escape_warning(yytext[1], yyscanner); addlitchar(unescape_single_char(yytext[1], yyscanner), yyscanner); } <xe>{xeoctesc} { unsigned char c = strtoul(yytext + 1, NULL, 8); check_escape_warning(yyscanner); addlitchar(c, yyscanner); if (c == '\0' || IS_HIGHBIT_SET(c)) yyextra->saw_non_ascii = true; } <xe>{xehexesc} { unsigned char c = strtoul(yytext + 2, NULL, 16); check_escape_warning(yyscanner); addlitchar(c, yyscanner); if (c == '\0' || IS_HIGHBIT_SET(c)) yyextra->saw_non_ascii = true; } <xq,xe,xus>{quotecontinue} { /* ignore */ } <xe>. { /* This is only needed for \ just before EOF */ addlitchar(yytext[0], yyscanner); } <xq,xe,xus><<EOF>> { yyerror("unterminated quoted string"); } {dolqdelim} { SET_YYLLOC(); yyextra->dolqstart = pstrdup(yytext); BEGIN(xdolq); startlit(); } {dolqfailed} { SET_YYLLOC(); /* throw back all but the initial "$" */ yyless(1); /* and treat it as {other} */ return yytext[0]; } <xdolq>{dolqdelim} { if (strcmp(yytext, yyextra->dolqstart) == 0) { pfree(yyextra->dolqstart); yyextra->dolqstart = NULL; BEGIN(INITIAL); yylval->str = litbufdup(yyscanner); return SCONST; } else { /* * When we fail to match $...$ to dolqstart, transfer * the $... part to the output, but put back the final * $ for rescanning. Consider $delim$...$junk$delim$ */ addlit(yytext, yyleng - 1, yyscanner); yyless(yyleng - 1); } } <xdolq>{dolqinside} { addlit(yytext, yyleng, yyscanner); } <xdolq>{dolqfailed} { addlit(yytext, yyleng, yyscanner); } <xdolq>. { /* This is only needed for $ inside the quoted text */ addlitchar(yytext[0], yyscanner); } <xdolq><<EOF>> { yyerror("unterminated dollar-quoted string"); } {xdstart} { SET_YYLLOC(); BEGIN(xd); startlit(); } {xuistart} { SET_YYLLOC(); BEGIN(xui); startlit(); } <xd>{xdstop} { char *ident; BEGIN(INITIAL); if (yyextra->literallen == 0) yyerror("zero-length delimited identifier"); ident = litbufdup(yyscanner); if (yyextra->literallen >= NAMEDATALEN) truncate_identifier(ident, yyextra->literallen, true); yylval->str = ident; return IDENT; } <xui>{dquote} { yyless(1); /* xuiend state looks for possible UESCAPE */ BEGIN(xuiend); } <xuiend>{whitespace} { /* stay in xuiend state over whitespace */ } <xuiend><<EOF>> | <xuiend>{other} | <xuiend>{xustop1} { /* no UESCAPE after the quote, throw back everything */ char *ident; int identlen; yyless(0); BEGIN(INITIAL); if (yyextra->literallen == 0) yyerror("zero-length delimited identifier"); ident = litbuf_udeescape('\\', yyscanner); identlen = strlen(ident); if (identlen >= NAMEDATALEN) truncate_identifier(ident, identlen, true); yylval->str = ident; return IDENT; } <xuiend>{xustop2} { /* found UESCAPE after the end quote */ char *ident; int identlen; BEGIN(INITIAL); if (yyextra->literallen == 0) yyerror("zero-length delimited identifier"); if (!check_uescapechar(yytext[yyleng - 2])) { SET_YYLLOC(); ADVANCE_YYLLOC(yyleng - 2); yyerror("invalid Unicode escape character"); } ident = litbuf_udeescape(yytext[yyleng - 2], yyscanner); identlen = strlen(ident); if (identlen >= NAMEDATALEN) truncate_identifier(ident, identlen, true); yylval->str = ident; return IDENT; } <xd,xui>{xddouble} { addlitchar('"', yyscanner); } <xd,xui>{xdinside} { addlit(yytext, yyleng, yyscanner); } <xd,xui><<EOF>> { yyerror("unterminated quoted identifier"); } {xufailed} { char *ident; SET_YYLLOC(); /* throw back all but the initial u/U */ yyless(1); /* and treat it as {identifier} */ ident = downcase_truncate_identifier(yytext, yyleng, true); yylval->str = ident; return IDENT; } {typecast} { SET_YYLLOC(); return TYPECAST; } {dot_dot} { SET_YYLLOC(); return DOT_DOT; } {colon_equals} { SET_YYLLOC(); return COLON_EQUALS; } {equals_greater} { SET_YYLLOC(); return EQUALS_GREATER; } {less_equals} { SET_YYLLOC(); return LESS_EQUALS; } {greater_equals} { SET_YYLLOC(); return GREATER_EQUALS; } {less_greater} { /* We accept both "<>" and "!=" as meaning NOT_EQUALS */ SET_YYLLOC(); return NOT_EQUALS; } {not_equals} { /* We accept both "<>" and "!=" as meaning NOT_EQUALS */ SET_YYLLOC(); return NOT_EQUALS; } {self} { SET_YYLLOC(); return yytext[0]; } {operator} { /* * Check for embedded slash-star or dash-dash; those * are comment starts, so operator must stop there. * Note that slash-star or dash-dash at the first * character will match a prior rule, not this one. */ int nchars = yyleng; char *slashstar = strstr(yytext, "/*"); char *dashdash = strstr(yytext, "--"); if (slashstar && dashdash) { /* if both appear, take the first one */ if (slashstar > dashdash) slashstar = dashdash; } else if (!slashstar) slashstar = dashdash; if (slashstar) nchars = slashstar - yytext; /* * For SQL compatibility, '+' and '-' cannot be the * last char of a multi-char operator unless the operator * contains chars that are not in SQL operators. * The idea is to lex '=-' as two operators, but not * to forbid operator names like '?-' that could not be * sequences of SQL operators. */ if (nchars > 1 && (yytext[nchars - 1] == '+' || yytext[nchars - 1] == '-')) { int ic; for (ic = nchars - 2; ic >= 0; ic--) { char c = yytext[ic]; if (c == '~' || c == '!' || c == '@' || c == '#' || c == '^' || c == '&' || c == '|' || c == '`' || c == '?' || c == '%') break; } if (ic < 0) { /* * didn't find a qualifying character, so remove * all trailing [+-] */ do { nchars--; } while (nchars > 1 && (yytext[nchars - 1] == '+' || yytext[nchars - 1] == '-')); } } SET_YYLLOC(); if (nchars < yyleng) { /* Strip the unwanted chars from the token */ yyless(nchars); /* * If what we have left is only one char, and it's * one of the characters matching "self", then * return it as a character token the same way * that the "self" rule would have. */ if (nchars == 1 && strchr(",()[].;:+-*/%^<>=", yytext[0])) return yytext[0]; /* * Likewise, if what we have left is two chars, and * those match the tokens ">=", "<=", "=>", "<>" or * "!=", then we must return the appropriate token * rather than the generic Op. */ if (nchars == 2) { if (yytext[0] == '=' && yytext[1] == '>') return EQUALS_GREATER; if (yytext[0] == '>' && yytext[1] == '=') return GREATER_EQUALS; if (yytext[0] == '<' && yytext[1] == '=') return LESS_EQUALS; if (yytext[0] == '<' && yytext[1] == '>') return NOT_EQUALS; if (yytext[0] == '!' && yytext[1] == '=') return NOT_EQUALS; } } /* * Complain if operator is too long. Unlike the case * for identifiers, we make this an error not a notice- * and-truncate, because the odds are we are looking at * a syntactic mistake anyway. */ if (nchars >= NAMEDATALEN) yyerror("operator too long"); yylval->str = pstrdup(yytext); return Op; } {param} { SET_YYLLOC(); yylval->ival = atol(yytext + 1); return PARAM; } {integer} { SET_YYLLOC(); return process_integer_literal(yytext, yylval); } {decimal} { SET_YYLLOC(); yylval->str = pstrdup(yytext); return FCONST; } {decimalfail} { /* throw back the .., and treat as integer */ yyless(yyleng - 2); SET_YYLLOC(); return process_integer_literal(yytext, yylval); } {real} { SET_YYLLOC(); yylval->str = pstrdup(yytext); return FCONST; } {realfail1} { /* * throw back the [Ee], and treat as {decimal}. Note * that it is possible the input is actually {integer}, * but since this case will almost certainly lead to a * syntax error anyway, we don't bother to distinguish. */ yyless(yyleng - 1); SET_YYLLOC(); yylval->str = pstrdup(yytext); return FCONST; } {realfail2} { /* throw back the [Ee][+-], and proceed as above */ yyless(yyleng - 2); SET_YYLLOC(); yylval->str = pstrdup(yytext); return FCONST; } {identifier} { //---------- 标识符 const ScanKeyword *keyword; char *ident; SET_YYLLOC(); /* Is it a keyword? */ //是否关键字? keyword = ScanKeywordLookup(yytext, yyextra->keywords, yyextra->num_keywords); if (keyword != NULL) { //是,则返回关键字值 yylval->keyword = keyword->name; return keyword->value; } /* * No. Convert the identifier to lower case, and truncate * if necessary. */ //如果不是关键字,则设置为小写字母,如需要则截断 ident = downcase_truncate_identifier(yytext, yyleng, true); yylval->str = ident; return IDENT; } {other} { SET_YYLLOC(); return yytext[0]; } <<EOF>> { SET_YYLLOC(); yyterminate(); } %%“PostgreSQL中的Rules有什么作用”的内容就介绍到这里了，感谢大家的阅读。如果想了解更多行业相关的知识可以关注亿速云网站，小编将为大家输出更多高质量的实用文章！
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