python - Expected behavior with regular expressions with capturing-groups in pandas' `str.extract()`

Question

I'm trying to get a grasp on regular expressions and I came across with the one included inside the str.extract method:

movies['year']=movies['title'].str.extract('.*((.*)).*',expand=True)

It is supposed to detect and extract whichever is in parentheses. So, if given this string: foobar (1995) it should return 1995. However, if I open a terminal and type the following

echo 'foobar (1995)` | grep '.*((.*)).*'

matches the whole string instead of only the content between parentheses. I assume the method is working with BRE flavor because of the parentheses scaping, and so is grep (default behavior). Also, regex matches in blue the whole string and green the year (capturing group). Am I missing something here? The regex works perfectly inside python

Answer 1

First of all, the behavior of Pandas .str.extract() is quite expected: it returns only the capturing group contents. The pattern used with extract requires at least 1 capturing group:

pat : string
Regular expression pattern with capturing groups

If you use a named capturing group, the new column will be named after the named group.

The grep command you provided can be reduced to

grep '((.*))'

as grep is capable of matching a line partially (does not require a full line match) and works on a per line basis: once a match is found the whole line is returned. To override that behavior, you may use -o switch.

With grep, you cannot return the capturing group contents. This can be worked around with PCRE regexp powered with -P option, but it is not available on Mac, for example. sed or awk may help in those situations, too.