How do I convert a String to a Number, catching invalid strings?

Question

I am trying to convert a String '1' to a Number (int) 1 in a vimL script. I have used the built-in command str2nr() so far, however I was not able to write this in an elegant and effective way.

Ideally I would like to catch invalid strings before attempting to convert them to an integer.

For example: 'abc1' will return 0 when run through the str2nr() command.

I probably need the right regex pattern to catch anything such as: 'a', 'a1b' '7xy' as an illegal value and anything like: '1' or '7' as a legal value, thus filtering out all non-integer values.

I tried something in the lines of:

let l:value = 'abcX1'
let l:debug = l:value =~ '\[0-9]^\[A-Z]\^[a-z]\' ? "YES" : "NO"
echom l:debug

This is expected to return a NO, while this:

let l:value = '7'
let l:debug = l:value =~ '\[0-9]^\[A-Z]\^[a-z]\' ? "YES" : "NO"
echom l:debug

is expected to return a YES.

However, as I said, I am not good with regular expressions and probably building the wrong pattern up in those examples above as they are not working.

Thanks.

Answer 1

You can represent the set of non-digit characters with the atom \D or with the expression [^0-9].

So, if your definition of an invalid string is any string containing a non-digit character, you could probably detect it like this:
let debug = value =~ '\D' ? "NO" : "YES"
or like this:
let debug = value =~ '[^0-9]' ? "NO" : "YES"

You can describe a set of characters with a pair of brackets [], and then put inside all the characters you want. You could imagine they're separated by the logical operator OR.

[012]  = 0 or 1 or 2
[0-9]  = 0 or 1 or 2 or ... 9
[^0-9] = anything except 0 or 1 or ... 9

A caret sign (^) positioned inside the pair of brackets just after the opening one allows you to invert the set of characters. It could be read as: anything except what follows.

The regex engine of Vim provides another kind of notation to describe some sets of characters which are then called atoms. For example:

• \d for digit, same as [0-9]
• \s for whitespace, a space or a tab
• \w for word character, same as [0-9A-Za-z_]

Usually you can "invert" an atom by using an uppercase character (there are some exceptions though):

• \D for non-digit, same as [^0-9]
• \S for non-whitespace, anything except a space or a tab
• \W for non-word character, same as [^0-9A-Za-z_]

If you want to know more about the various sets of characters or atoms you can use to build your regexes, see:

:help /character-classes