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Assuming that the "WBA" spot will be a sequence of any letter or number, followed by a space, dash, and space:
str = "WBA - Skinny Joe vs. Hefty Hal"
str.sub /^\w+\s-\s/, ''
By the way — RegexPal is a great tool for testing regular expressions like these.
If you need a more complex string replacement, you can look into writing a more sophisticated regular expression. Otherwise:
Keep it simple! If you only need to remove "WBA - " from the beginning of the string, use String#sub.
s = "WBA - Skinny Joe vs. Hefty Hal"
puts s.sub(/^WBA - /, '')
# => Skinny Joe vs. Hefty Hal