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POHow to make lex/flex recognize tokens not separated by whitespace?
 

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  1. POHow to make lex/flex recognize tokens not separated by whitespace?
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    <p>I'm taking a course in compiler construction, and my current assignment is to write the lexer for the language we're implementing. I can't figure out how to satisfy the requirement that the lexer must recognize concatenated tokens. That is, tokens not separated by whitespace. E.g.: the string <code>39if</code> is supposed to be recognized as the number <code>39</code> and the keyword <code>if</code>. Simultaneously, the lexer must also <code>exit(1)</code> when it encounters invalid input.</p> <p>A simplified version of the code I have:</p> <pre><code>%{ #include &lt;stdio.h&gt; %} %option main warn debug %% if | then | else printf("keyword: %s\n", yytext); [[:digit:]]+ printf("number: %s\n", yytext); [[:alpha:]][[:alnum:]]* printf("identifier: %s\n", yytext); [[:space:]]+ // skip whitespace [[:^space:]]+ { printf("ERROR: %s\n", yytext); exit(1); } %% </code></pre> <p>When I run this (or my complete version), and pass it the input <code>39if</code>, the error rule is matched and the output is <code>ERROR: 39if</code>, when I'd like it to be:</p> <pre><code>number: 39 keyword: if </code></pre> <p>(I.e. the same as if I entered <code>39 if</code> as the input.)</p> <p><a href="http://westes.github.io/flex/manual/Matching.html" rel="nofollow noreferrer">Going by the manual</a>, I have a hunch that the cause is that the error rule matches a longer possible input than the number and keyword rules, and flex will prefer it. That said, I have no idea how to resolve this situation. It seems unfeasible to write an explicit regexp that will reject all non-error input, and I don't know how else to write a "catch-all" rule for the sake of handling lexer errors.</p> <p><strong>UPDATE:</strong> I suppose I could just make the catch-all rule be <code>. { exit(1); }</code> but I'd like to get some nicer debug output than "I got confused on line 1".</p>
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    1. COa) Have you run your simplified version? b) what does it do that is wrong?
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    2. CO@IraBaxter Sorry, seems I forgot to be explicit about my test case while lost in the speculation in the last paragraph. The answers are **a)** yes; and **b)** reports the lexer error instead of two tokens. (I've also added them into the question.)
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    3. COAh. OK, yes, your "^space" rule will eat any sequence of non-space, and thus consume "39if". Secret: avoid rules whose regexes overlap, unless the longer rule comes safely first. In your case, I'd use (I'm not a lex-pert) something to replace :^space: that was "not a digit, not a letter, not a space". ...
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