regex a period in zsh

I am trying to find out if a hostname has a domain suffix by searching for a period in the string. I can do it in bash with the following

#!/bin/bash
if [[ "$1" =~ \. ]]; then
  host="$1"
  echo "keep as $host"
else
  host="$1.mydomain.biz"
  echo "change to $host"
fi

but it doesn't work for zsh. The [[ "$1" =~ \. ]] always evaluates to True:

#!/bin/zsh
if [[ "$1" =~ \. ]]; then
  # always evaluates to True in zsh
  host="$1"
  echo "keep as $host"
else
  host="$1.mydomain.biz"
  echo "change to $host"
fi

Eventually something like this will go in my .zshrc, but experimenting with it as a shell file gave me insight.

How should I update the regex evaluation to get this to work in zsh? Or is there a different test I can use to determine if I need to add the domain suffix?

My environment is macOS Sonoma 14.2.1, zsh 5.9 (x86_64-apple-darwin23.0)

Solution

There fix is that you need to double escape the dot here:

if [[ "$1" =~ \\. ]]; then

Then the backslash will be treated as a regular, literal backslash that escapes a dot in the regex pattern.

Else, you may simply use

if [[ "$1" == *.* ]]; then

where * stands for any text (it is required because == requires an exact match, full string match), but that is no longer a regex.