rex mode=sed does not allow arbitrary delimiters

[StrongestNumber9]

Describe the bug

Can't use arbitrary delimiter Invalid sed mode string was given: 's,fluffy,cute,g', but expected 's/original/replacement/g'

Expected behavior

Expecting rex mode=sed to use sed-like arguments as in the first letter after s is the delimiter

$ echo 'fluffy cat' | sed 's,fluffy,cute,g'
cute cat

How to reproduce

rex mode=sed "s,a,b,g"

Screenshots

Software version

Desktop (please complete the following information if relevant):

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Additional context

[eemhu]

internal pr merged