Answer to regex golf http://regex.alf.nu/

rg.py

plain = r'foo'
anchor = r'k$'
ranges = r'^[a-f]*$'
backrefs = r'(...).*\1'

abba = (r'^(?!' # don't (
        r'.*(.)(.)\2\1.*'  # parttern like abba
        r'$)')  # ) select

plan = r'^(.)(.).*\2\1$'

prime = (r'^(?!' # don't (
         r'(xx+?)\1+'  # parttern that 'x' group (length > 1) is repeated
         r'$)')  # ) select

four = r'(.).\1.\1.\1'

order = r'^[a-h]*[i-z]*$'  # cheat a bit, try to find a good split spot

triples = 'link: http://quaxio.com/triple/'  # tough, see this blog that
                                             # construct a FSM first

glob = (r'^(\w*) matches \1$|'  # no star
        r'^(\w*)\*(\w*) matches \2\w+\3$|'  # one star
        r'^(\w*)\*(\w*)\*(\w*) matches \4\w+\5\w+\6$|'  # two star
        r'^(\w*)\*(\w*)\*(\w*)\*(\w*) matches \7\w+\8\w+\9\w+\10$')
        
glob2 = (r'^(\*?)(\w*)(\*?)(\w*)(\*?)(\w*)'
         r' .* '  # matches
         r'((.(?!\1))+|\1)\2((.(?!\3))+|\3)\4((.(?!\5))+|\5)\6$')
# ((.(?!\1))+|\1) is used to conditionally match .+ if \1 if found (via @hadrel)

balance = r'^(<(<(<(<(<(<(<>)*>)*>)*>)*>)*>)*>)*$'

powers = (r'^(?!' # don't (
          r'(x(xx)+)'  # parttern that odd number of 'x'
          r'\1*'       # is repeated
          r'$)')  # ) select (via @plby)

Nooo!
Wish I hadn't seen this. Must. not. cheat.

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