Selecting different ranges in a giant dataframe in RStudio


I have a VERY big csv with dates of several actions and their closing prices, impossible to use Excel.
The name of the action is in the same column of the date and only appears at the beginning of the series, according to the following image:
insert the description of the image here

I have limited knowledge in R and I am in need of some function to help me do this interval reading.
NOTE: The name of the action is always in parentheses: (ACTION X)

Can somebody help me?