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expr      := number
           | expr ws unary_op 
...
unary_op  := "chs" | "abs" | "sqr" | "sqrt" | "sin" | "cos" | "tan"
binary_op := '+' | '-' | '*' | '/' | '^'
ws        := ' ' | '\t' | '\r' | '\n'

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```
Infinite recursion in regex
```

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expr      := number
           | expr ws unary_op 
           | expr ws expr ws binary_op

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expr      := number expr_
expr_     := ws expr ws binary_op expr_
           | ws unary_op expr_
           |

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qr{
   (?(DEFINE)
...
   \A \s* (?&expr) \s* \z
   (?{ $result = pop(@stack) })
}sx;

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#!/usr/bin/perl

...
    my ($expression) = $line =~ /($RPN)/;
    ...
}

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OK: .456 = .456
OK: 123. = 123.
...
OK: 4 2 / = 2
OK: 24abssqrt is invalid, got no result
OK: 4 2 * = 8