Use regex test instead of match for existence

[rtibbles]

Summary

Description of the change(s) you made

Implements recommended performance improvement from https://deepsource.io/gh/learningequality/studio/issue/JS-D007/occurrences?page=1

Uses regex test instead of match when the results of the match are not needed

Reviewer guidance

How can a reviewer test these changes?

Does Markdown editing still work as expected?

From reading the code, this seems very low risk to me (except that when I originally changed the code, I just changed match to test, but had to change it so that I was calling the method on the regexp, rather than the string).

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Contributor's Checklist

PR process:

• If this is an important user-facing change, PR or related issue the CHANGELOG label been added to this PR. Note: items with this label will be added to the CHANGELOG at a later time
• If this includes an internal dependency change, a link to the diff is provided
• The docs label has been added if this introduces a change that needs to be updated in the user docs?
• If any Python requirements have changed, the updated requirements.txt files also included in this PR
• Opportunities for using Google Analytics here are noted
• Migrations are safe for a large db

Studio-specifc:

• All user-facing strings are translated properly
• The notranslate class been added to elements that shouldn't be translated by Google Chrome's automatic translation feature (e.g. icons, user-generated text)
• All UI components are LTR and RTL compliant
• Views are organized into pages, components, and layouts directories as described in the docs
• Users' storage used is recalculated properly on any changes to main tree files
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Testing:

• Code is clean and well-commented
• Contributor has fully tested the PR manually
• If there are any front-end changes, before/after screenshots are included
• Critical user journeys are covered by Gherkin stories
• Any new interactions have been added to the QA Sheet
• Critical and brittle code paths are covered by unit tests

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Reviewer's Checklist

This section is for reviewers to fill out.

• Automated test coverage is satisfactory
• PR is fully functional
• PR has been tested for accessibility regressions
• External dependency files were updated if necessary (yarn and pip)
• Documentation is updated
• Contributor is in AUTHORS.md

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!

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