Martin Panter added the comment:
Okay so it sounds like my usage is valid if I add the brackets. I will try to
come up with a documentation patch as some stage. The reason why it is not
supported without brackets is to maintain a bit of consistency with the
question mark (?), which modifies the preceding quantifier, and with the plus
sign (+), which is also a modifier in other implementations.
For the record, Gnu grep does seem to accept my expression (although Posix says
this is undefined, and neither support lazy or possessive quantifiers):
$ grep -E -o 'a{2}*' <<< "aaaaa"
aaaa
However pcregrep, which supports lazy (?) and possessive (+) quantifiers,
doesn’t like my expression:
$ pcregrep -o 'a{2}*' <<< "aaaaa"
pcregrep: Error in command-line regex at offset 4: nothing to repeat
[Exit 2]
$ pcregrep -o '(?:a{2})*' <<< "aaaaa"
aaaa
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