title: replace substring in pandas data frame column
body: <p>I am working with dataframe that contains column named "raw_parameter_name". In this column i have different string values. Several values are like following pattern "ABCD;MEAN". 
  What i am trying to do is to replace each value "ABCD;MEAN" with "ABCD;X-BAR". Sub string "ABCD" may vary but pattern ";MEAN" is constant i want to replace. Looked into different options using "replace" method but don't know how to replace sub string only and not whole string. Please advise.
  Thank you in advance</p>