username length character limits?

Please CC me:

If I create the user 'web_user'
with password 'password'
I can connect using 'psql' just fine.

If I create the user 'D1Khb2g5m7FGk_web_user'
with password 'password'
I CANNOT connect using 'psql', I get authentication error.

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ANYONE have any ideas why?
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PG_VERSION says it's 7.3. I don't know how to find out any more accurately the subversion of that.