# reccalc - recursive calculator with Flex and Bison
In this example the generated parser is pure and reentrant: it can be used
concurrently in different threads, or recursively. As a proof of this
reentrancy, expressions in parenthesis are tokenized as strings, and then
recursively parsed from the parser:
```
exp: STR
{
result r = parse_string ($1);
free ($1);
if (r.nerrs)
{
res->nerrs += r.nerrs;
YYERROR;
}
else
$$ = r.value;
}
```
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