当然 Regexp 对此有好处:

string = "123abcd"
/^(?<num>\d+)$/ =~ string
num # => nil

string = "123"
/^(?<num>\d+)$/ =~ string
num # => '123' # String

所以如果你需要检查条件:

So if you need to check the condition:

if /^(?<num>\d+)$/ =~ string
   num.to_i # => 123
   # do something...
end

#to_i String 的方法对您的情况无效，因为它会返回一个数字，如果字符串与字母是偶数:

#to_i method of String isn't valid for your case because it will return a number, if string is even with letters:

string = "123abcd"
string.to_i # 123