regex - PANDAS Finding the exact word and before word in a column of string and append that new column in python (pandas) column

Question

Find the target word and the before word in col_a and append matched string in col_b_PY and col_c_LG columns

    This code i have tried to achive this functionality but not able to 
get the expected output. if any help appreciated
Here is the below code i approach with regular expressions:

df[''col_b_PY']=df.col_a.str.contains(r"(?:[a-zA-Z'-]+[^a-zA-Z'-]+) 
{0,1}PY")

df.col_a.str.extract(r"(?:[a-zA-Z'-]+[^a-zA-Z'-]+){0,1}PY",expand=True)

Dataframe looks like this

col_a

Python PY is a general-purpose language LG

Programming language LG in Python PY 

Its easier LG to understand  PY

The syntax of the language LG is clean PY 

Desired output:

col_a                                       col_b_PY      col_c_LG
Python PY is a general-purpose language LG  Python PY     language LG

Programming language LG in Python PY        Python PY     language LG

Its easier LG to understand  PY            understand PY easier LG

The syntax of the language LG is clean PY   clean  PY     language LG

Answer 1

You may use

df['col_b_PY'] = df['col_a'].str.extract(r"([a-zA-Z'-]+s+PY)")
df['col_c_LG'] = df['col_a'].str.extract(r"([a-zA-Z'-]+s+LG)")

Or, to extract all matches and join them with a space:

df['col_b_PY'] = df['col_a'].str.extractall(r"([a-zA-Z'-]+s+PY)").unstack().apply(lambda x:' '.join(x.dropna()), axis=1)
df['col_c_LG'] = df['col_a'].str.extractall(r"([a-zA-Z'-]+s+LG)").unstack().apply(lambda x:' '.join(x.dropna()), axis=1)

Note you need to use a capturing group in the regex pattern so that extract could actually extract the text:

Extract capture groups in the regex pat as columns in a DataFrame.

Note the word boundary is necessary to match PY / LG as a whole word.

Also, if you want to only start a match from a letter, you may revamp the pattern to

r"([a-zA-Z][a-zA-Z'-]*s+PY)"
r"([a-zA-Z][a-zA-Z'-]*s+LG)"
   ^^^^^^^^          ^

where [a-zA-Z] will match a letter and [a-zA-Z'-]* will match 0 or more letters, apostrophes or hyphens.

Python 3.7 with Pandas 0.24.2:

pd.set_option('display.width', 1000)
pd.set_option('display.max_columns', 500)

df = pd.DataFrame({
    'col_a': ['Python PY is a general-purpose language LG',
             'Programming language LG in Python PY',
             'Its easier LG to understand  PY',
             'The syntax of the language LG is clean PY',
             'Python PY is a general purpose PY language LG']
    })
df['col_b_PY'] = df['col_a'].str.extractall(r"([a-zA-Z'-]+s+PY)").unstack().apply(lambda x:' '.join(x.dropna()), axis=1)
df['col_c_LG'] = df['col_a'].str.extractall(r"([a-zA-Z'-]+s+LG)").unstack().apply(lambda x:' '.join(x.dropna()), axis=1)

Output:

                                           col_a              col_b_PY     col_c_LG
0     Python PY is a general-purpose language LG             Python PY  language LG
1           Programming language LG in Python PY             Python PY  language LG
2                Its easier LG to understand  PY        understand  PY    easier LG
3      The syntax of the language LG is clean PY              clean PY  language LG
4  Python PY is a general purpose PY language LG  Python PY purpose PY  language LG