Update variable regex to support hyphens

[stikkireddy]

Changes

Modified interpolation logic to use: \$\{([a-zA-Z]+([-_]*[a-zA-Z0-9]+)*(\.[a-zA-Z]+([-_]*[a-zA-Z0-9]+)*)*)\}

Edit: Suggested by @pietern \$\{([a-zA-Z]+([-_]?[a-zA-Z0-9]+)*(\.[a-zA-Z]+([-_]?[a-zA-Z0-9]+)*)*)\} to be more selective and not allow consequent hyphens or underscores to make the keys more readable.

Explanation:

1. All interpolation starts with ${ and ends with }
2. All interpolated locations are split by by .
3. All sections are expected to start with a alphabet [a-zA-Z]; no numbers, hyphens or underscores.
4. All sections are expected to end with an alphanumeric [a-zA-Z0-9] no hyphens or underscores

This change allows the current interpolation to be more permissive.

Note it does break backwards compatibility because [a-zA-Z] != [\w]. \w includes alphanumeric and underscores. \w = [a-zA-Z0-9_]

Tests

There are two tests with examples of valid and invalid interpolation and a test to validate expansion.

[pietern]

WDYT?