Flex & bison by John R Levine; Safari Tech Books Online

By John R Levine; Safari Tech Books Online

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It matches a string of letters, [a-z]+, optionally followed by an apostrophe and either s or t, to match words such as owner’s and can’t. Each matched word Symbol Tables and a Concordance Generator | 33 is passed to addref(), described in a moment, along with the current filename and line number. The final pattern is a catchall to match whatever the previous patterns didn’t. Note that this scanner is extremely ambiguous, but flex’s rules for resolving ambiguity make it do what we want. It prefers longer matches to shorter ones, so the word toad will be matched by the main word pattern, not to.

The action in the second rule prints the value of the exp in $2. The rest of the rules implement the calculator. The rules with operators such as exp ADD factor and ABS term do the appropriate arithmetic on the symbol values. The rules with a single symbol on the right side are syntactic glue to put the grammar together; for example, an exp is a factor. In the absence of an explicit action on a rule, the parser assigns $1 to $$. This is a hack, albeit a very useful one, since most of the time it does the right thing.

The third rule matches everything in between. Although the complexity of patterns doesn’t affect the speed of a flex scanner, it is definitely faster to match one big pattern than several little ones. |\n, but the ([^*]|\n)+ can match a long string of text at once. Note that it has to exclude * so that the second rule can match */. The <> rule catches and reports unterminated comments. Next is a bonus rule that matches C++-style comments, a common extension to C compilers. Patterns for C Comments Although it’s possible to match C comments with a single flex pattern, it’s generally not a great idea to do so.

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