Currently as the first column in a data frame I have string values in the 
format xx_yy - I want to create a new column with just the substring xx (for 
each row in turn).  Three possible ways to do this might be (1) split the 
string by '_' using strsplit and paste the first of the resulting variables 
into a new column, but I have been unable to do this for each row of my data 
frame in turn (trying to use apply); (2) split the column into two based on 
'_', but I am not sure if this is possible; (3) use a regular expression to 
return the substring up to the '_', but I am unsure how to make a regular 
expression return the substring it matches to in R.

Any ideas on all three counts would be gratefully recieved.

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