问题
I'm trying to use a regular expression to solve a reverse polish calculator problem, but I'm having issues with converting the mathematical expressions into conventional form.
I wrote:
puts '35 29 1 - 5 + *'.gsub(/(\d*) (\d*) (\W)/, '(\1\3\2)')
which prints:
35 (29-1)(+5) *
expected
(35*((29-1)+5))
but I'm getting a different result. What am I doing wrong?
回答1:
I'm assuming you meant you tried
puts '35 29 1 - 5 + *'.gsub(/(\d*) (\d*) (\W)/, '(\1\3\2)')
^ ^
Anyway, you have to use the quantifier + instead of *, since otherwise you will match an empty string for \d* as one of your captures, hence the (+5):
/(\d+) (\d+) (\W)/
I would further extend/constrain the expression to something like:
/([\d+*\/()-]+)\s+([\d+*\/()-]+)\s+([+*\/-])/
| | | | |
| | | | Valid operators, +, -, *, and /.
| | | |
| | | Whitespace.
| | |
| | Arbitrary atom, e.g. "35", "(29-1)", "((29-1)+5)".
| |
| Whitepsace.
|
Arbitrary atom, e.g. "35", "(29-1)", "((29-1)+5)".
...and instead of using gsub, use sub in a while loop that quits when it detects that no more substitutions can be made. This is very important because otherwise, you will violate the order of operations. For example, take a look at this Rubular demo. You can see that by using gsub, you might potentially replace the second triad of atoms, "5 + *", when really a second iteration should substitute an "earlier" triad after substituting the first triad!
WARNING: The - (minus) character must appear first or last in a character class, since otherwise it will specify a range! (Thanks to @JoshuaCheek.)
来源：https://stackoverflow.com/questions/12293395/regexp-issue-involving-reverse-polish-calculator