cheat sheets.

To change case: 

 capitalize - first character to upper, rest to lower
 downcase - all to lower case
 swapcase - changes the case of all letters
 upcase - all to upper case

To rejustify: 

 center - add white space padding to center string
 ljust - pads string, left justified
 rjust - pads string, right justified

To trim: 

 chop - remove last character
 chomp - remove trailing line separators
 squeeze - reduces successive equal characters to singles
 strip - deletes leading and trailing white space

To examine: 

 count - return a count of matches
 empty? - returns true if empty
 include? - is a specified target string present in the source?
 index - return the position of one string in another
 length or size - return the length of a string
 rindex - returns the last position of one string in another
 slice - returns a partial string

To encode and alter: 

 crypt - password encryption
 delete - delete an intersection
 dump - adds extra \ characters to escape specials
 hex - takes string as hex digits and returns number
 next or succ - successive or next string (eg ba -> bb)
 oct - take string as octal digits and returns number
 replace - replace one string with another
 reverse - turns the string around
 slice! - DELETES a partial string and returns the part deleted
 split - returns an array of partial strings exploded at separator
 sum - returns a checksum of the string
 to_f and to_i - return string converted to float and integer
 tr - to map all occurrences of specified char(s) to other char(s)
 tr_s - as tr, then squeeze out resultant duplicates
 unpack - to extract from a string into an array using a template

To iterate: 

 each - process each character in turn
 each_line - process each line in a string
 each_byte - process each byte in turn
 upto - iterate through successive strings (see "next" above)

Taken from: http://www.wellho.net/solutions/ruby-string-functions-in-ruby.html