【正文】 will be inactive. ? If the start condition is inclusive, then rules with no start conditions at all will also be active. ? If it is exclusive, then only rules qualified with the start condition will be active. %s example %% examplefoo do_something()。假如 yywrap() 函式傳回 1,則字彙分析器傳回一個 token 0 來代表遇到檔案結尾 ? 在 lex 函式庫中的標準 yywrap() 函式永遠會傳回 1, 但是你可以用自己寫的來代替它 .假如 yywrap() 函式傳回 0, 表示還有其它的輸入資料 , 這個時候需要先重新設定 yyin 指向新的檔案 (用 fopen 來設定 ) ? 在我們的 lex 輸入檔中 , 我們定義 yywrap() 永遠回傳 1, 表示只有一個檔案需要處理 yyinput(), yyunput() ? flex 提供 yyinput() 以及 yyunput() 來包裝 input(), unput(). ? unput(c) 函式會將字元 c 放回輸入資料中 . 和一般 stdio 中 unputc()函式不同的是 : 你可以連續(xù)呼叫 unput() 來將一堆字元放回去 . yyless(), yymore() ? 在動作程式碼中呼叫 yyless(n), 會將該規(guī)則配對到的 token 保留前 n 個字元 , 其它的則 “放” 回去 . 在判斷 token 的邊界時 , 而且又不容易表示成常規(guī)表示法時很有用 . yyless 和 yymore 可搭配使用 , 利用 yymore 來告訴 lex 將下一個 token 附加到目前的 token 上 extract string literal “abc\”mac” “abc\” ? \[^]*\ 傳回最後一個引號 加入下一個字串 \[^]*\ “ a b c \ ” m a c ” input buffer regular expression yytext “ \[^]*\ “ a b c \ ” m a c ” “ a \[^]*\ “ a b c \ ” m a c ” “ a b \[^]*\ “ a b c \ ” m a c ” “ a b c \[^]*\ “ a b c \ ” m a c ” “ a b c \ Analyzing process [1] input buffer regular expression yytext Analyzing process [2] \[^]*\ “ a b c \ ” m a c ” “ a b c \ “ yyleng = 6 “ a b c \ ” m a c ” “ a b c \ ” m a c ” \[^]*\ “ a b c \ “ “ a b c \ “ a b c \ ” m a c ” \[^]*\ “ a b c \ “ m “ a b c \ ” m a c ” \[^]*\ “ a b c \ “ m a unput character ” input buffer regular expression yytext Analyzing process [3] “ a b c \ ” m a c ” \[^]*\ “ a b c \ “ m a c “ a b c \ ” m a c ” \[^]*\ “ a b c \ “ m a c “ fails “ a b c \ “ m a c “ yytext \0 yyleng = 10 Starting condition (開始狀態(tài) ) ? flex provides a mechanism for conditionally activating rules. Any rule whose pattern is prefixed with sc will only be active when the scanner is in the start condition named sc. ? Start conditions are declared in the definitions (first) section of the input using unindented lines beginning with either `%s39。 或 extern char *yytext 。 Does “g++ ” work? Q4: can we pile in ? [1] Download and in Q3 into local machine Error occurs when piling VC does not have this header file Q4: can we pile in ? [2] /usr/include/ Q4: can we pile in ? [3] disable “” in Error occurs since prototype of function isatty is declared in /usr/include/ Q4: can we pile in ? [4] OutLine ? What is lex ? Regular expression ? Finite state machine ? Content of flex ? Application Regular expression ? A regular expression, often called a pattern, is an expression that describes a set of strings. ? The origins of regular expressions lie in automata theory and formal language theory, both of which are part of theoretical puter science . In the 1950s, mathematician Stephen Cole Kleene described these models using his mathematical notation called regular sets. ? Most formalisms provide the following operations to construct regular expressions alternation: A vertical bar separates alternatives. For example, gray|grey can match “gray” or “grey”. grouping: use parentheses to define the scope and precedence of the opera