Error with regex in `Str::placeholders`

Start with the placeholders method in bolt/common and reproduce the failure using the Bolt/Forms notification configuration and fields shown in the issue. Verify the regular expression's handling of replyto_email: email and replyto_name: lastname; done means the form renders without the RFC 2822 validation error.

Hello,

I'm reporting an error I detected when I wanted to use in Bolt/Forms extension, parameters replyto_email and replyto_name.

The form returns me an error:

"An exception has been thrown during the rendering of a template ("Email "email" does not comply with addr-spec of RFC 2822.")."

After debugging, it turns out that the error comes from bolt/common, in the placeholders method.

The regexp doesn't seem to be good at detecting the string sent to it.

Example to reproduce the bug:

1. Install last bolt version
2. Install last bolt/forms version
3. Create form with notification parameters like that :

notification:
        enabled: true
        debug: false
        debug_address: name@example.com # Email address used when debug mode is enabled
        debug_smtp: true
        subject: New contact
        subject_prefix: '[XXX]'
        to_name: 'John'
        to_email: 'john.doe@example.com'
        from_name: 'James'
        from_email: 'james.bond@example.com'
        replyto_name: lastname
        replyto_email: email

4. Add two fields

lastname:
            type: text
            fieldLabel: Nom
            options:
                required: true
                label: Lastname
                constraints: [ NotBlank ]
                attr:
                    placeholder: Nom
email:
            type: email
            fieldLabel: Email
            options:
                required: true
                constraints: [ NotBlank, Email ]
                attr:
                    placeholder: Email

5. Test form
6. Error appear

Here a pr to fix it (works for me)