Usage of the =~ operator

Question

I'm a little confused on VimScript's regex comparison operators.

If I look through :h =~? my understanding is that =~ can be used to do a regex match (with the ? making it case-insensitive).

But this code prints "no":

let s:myVar = "S123"
if s:myVar =~? "^S\d+"
  echo "yes"
else
  echo "no"
endif

Am I using the =~ operator incorrectly?

Answer 1

There are two problems:

1. You need \+ since you're not using magic.

2. You're using " instead of '. You need to double escape \: \\d, or use single quotes.

   See :h literal-string:

   literal-string                                          literal-string E115
   ---------------
   'string'                string constant                 expr-'
   
   Note that single quotes are used.
   
   This string is taken as it is.  No backslashes are removed or have a special
   meaning.  The only exception is that two quotes stand for one quote.
   
   Single quoted strings are useful for patterns, so that backslashes do not need
   to be doubled.  These two commands are equivalent: 
           if a =~ "\\s*"
           if a =~ '\s*'
   

So:

let s:myVar = "S123"
if s:myVar =~? '^S\d\+'
  echo "yes"
else
  echo "no"
endif

This works as expected.