如何在正则表达式中匹配' - '在[..]内部

Question

I am trying to match a '-' inside a [..] block using regular expressions in python, but, I am not sure how to make that happen, since '-' denotes ranges in that block.

Edit: my failing regex:

regex = re.compile("^[0-9+-*/]+$")

Answer 1

From the docs :

If you want to include a ']' or a '-' inside a set, precede it with a backslash, or place it as the first character.

Answer 2

Just place it at the beginning of the [] (character class):

regex = re.compile("^[-0-9+*/]+$")

Why does it work?

When you place the hyphen at the beginning of the character class, most regular expression engines are intelligent enough to realize that you mean a literal hyphen (since you can't indicate a range without an beginning).