Use regular expressions
(http://www.ruby-doc.org/docs/ProgrammingRuby/html/intro.html#S5), then
#scan the string for something that matches. Eg. assuming the format is
always 'treeframe' followed by digits:

irb(main):038:0> c.scan /treeframe\d+/
=> ["treeframe1120266500902"]

You'll get an array with all the results. If you know you have only one
occurence you can use String#slice (or String#[]) to get the first
value:

irb(main):037:0> c[/treeframe\d+/]
=> "treeframe1120266500902"

HTH,
Assaph