title: replacing a column in DataFrame using regex
body: <p>I have a data frame with 4 columns, col4 is a string including texts and digits:</p>
  
  <pre><code> Col1           Col2              Col3           Col4
  Syslog        2016,09,17           1    PD380_003 %LINK-3-UPDOWN
  Syslog        2016,09,17           1    NM380_005 %BGP-5-NBR_RESET
  Syslog        2016,09,14           1    NM380_005 %BGP-5-NBR_RESET
  Syslog        2016,09,08           1    DO NOT TICKET LO380_004 %SYS-5-CONFIG_I Config
  </code></pre>
  
  <p>i need to keep a substring of that column and delete anything else, so I used regex and i made a pattern but when i run the following query the result is not what i want, it replace everything with the pattern itself:</p>
  
  <pre><code>data.replace({'Col4':{'.*':'([A-Z]{2}[0-9]{3}_[0-9]{3})'}},regex=True)
  </code></pre>
  
  <p>desired result is:</p>
  
  <pre><code> Col1           Col2              Col3           Col4
  Syslog        2016,09,17           1           PD380_003
  Syslog        2016,09,17           1           NM380_005
  Syslog        2016,09,14           1           LO380_004
  Syslog        2016,09,08           1           LO380_004
  </code></pre>
  
  <p>but the result i get is like:</p>
  
  <pre><code> Col1           Col2              Col3           Col4
  Syslog        2016,09,17           1    ([A-Z]{2}[0-9]{3}_[0-9]{3})
  Syslog        2016,09,17           1    ([A-Z]{2}[0-9]{3}_[0-9]{3})
  Syslog        2016,09,14           1    ([A-Z]{2}[0-9]{3}_[0-9]{3})
  Syslog        2016,09,08           1    ([A-Z]{2}[0-9]{3}_[0-9]{3})
  </code></pre>
  
  <p>what am i doing wrong?</p>