How to remove some characters from text?

You can use the regex plugin, or the substitute function.

Use These:

• substitute() for the exact string "<a></a>" or "<a></a>".
• regexreplace() if you need to remove any <a ...>...</a> tag regardless of attributes or contents

• If your text contains escaped HTML entities
  substitute(
    ri!text,
    "<a></a>",
    ""
  )