You didn't ask to match an "is" at the start of the string or an "is" at the end of the string. You asked to match one that is both.

>perl -le"$_='is'; s/^is$/are/g; print"
are

>perl -le"$_='is is is'; s/^is$/are/g; print"
is is is

>perl -le"$_='is is is'; s/^is|is$/are/g; print"
are is are
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