Fix parse failures with REGEX frontend due to white space in declarations

[reuterbal]

This fixes #318. Thanks @piotrows for reporting this and providing a small reproducer.

[github-actions[bot]]

Documentation for this branch can be viewed at https://sites.ecmwf.int/docs/loki/323/index.html

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 95.15%. Comparing base (77114a9) to head (ae54830).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #323 +/- ## ======================================= Coverage 95.14% 95.15% ======================================= Files 167 168 +1 Lines 35374 35404 +30 ======================================= + Hits 33657 33687 +30 Misses 1717 1717 ``` | [Flag](https://app.codecov.io/gh/ecmwf-ifs/loki/pull/323/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ecmwf-ifs) | Coverage Δ | | |---|---|---| | [lint_rules](https://app.codecov.io/gh/ecmwf-ifs/loki/pull/323/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ecmwf-ifs) | `96.38% <ø> (ø)` | | | [loki](https://app.codecov.io/gh/ecmwf-ifs/loki/pull/323/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ecmwf-ifs) | `95.12% <100.00%> (+<0.01%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ecmwf-ifs#carryforward-flags-in-the-pull-request-comment) to find out more.

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