grep -i "something ~\* '[[:alnum:]]*'" /var/log/syslog 

works for me.

• escape the first * to match a literal * instead of making it the zero-or-more-matches character
• use double brackets around :alnum: (see example here)
• use a * after [[:alnum::]] to match not only one character between your single quotes but several of them

grep -i "something ~\* '[[:alnum:]]*'" /var/log/syslog 

works for me.

• escape the first * to match a literal * instead of making it the zero-or-more-matches character:
  ~* would match zero or more occurrences of ~ while
  ~\* matches the expression ~* after something
• use double brackets around :alnum: (see example here)
• use a * after [[:alnum::]] to match not only one character between your single quotes but several of them
• the single quotes don't have to be escaped at all because they are contained in an expression that is limited by double quotes.