I have a program that I use for this sort of thing, specifically written always to generate a single regular expression.

The program is 'word', and it includes flags -a for "anagram", -u for "use any subset of the letters", and -D for "just show me the regexp".

For an exact anagram, word -Da rtesamcna gives:

/^(?:(?=.*e)(?=.*n)(?=.*c)(?=.*r)(?=.*a.*a)(?=.*m)(?=.*s)(?=.*t).{9,9}
+)\z/oi
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That is: require each of the single letters, require a double "a", and require exactly 9 letters.

For all subsets, word -Dua rtesamcna gives:

/^(?:(?:([encrmst])(?!.*\1)|([a])(?!.*\2.*\2))+)\z/oi
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That is: require all matching letters to be from the list, and additionally don't allow more than one of any of [encrmst], nor more than two "a"s.

(Note: normal use of the word program also requires wrap from the same place.)

Hugo