Regular expression pattern to replace in Python

Question

Apologies if a similar question is answered already. But here is what I am struggling with, to get a regular expression replace for.

My input text has this sample

" var1 || literal "

I need a regular expression sub to something like this

" concat var1, literal "

Essentially, || should be replaced with a comma and the first element should be prefixed with a "concat". I may have multiple such occurances in a given input and so I should replace everywhere.

Here is where I am stuck. I can build the regex pattern, but I am not sure how to substitute it and with that.

re.sub(r'\s{1}[a-zA-Z0-9_]+\s*\|\|\s*[a-zA-Z0-9_]+\s*, '??????', input_string)

I am not sure if this can be done in a single Python statement.

I have an alternative to run through the string in loop and get each occurance and replace it individually without using regular expression.

Thanks in advance. Radha

Answer 1

You may handle this requirement using re.sub with a callback function:

sql = "select var1 || literal || var2 from yourTable"
def to_concat(matchobj):
    return "concat(" + re.sub(r'\s*\|\|', ',', matchobj) + ")"

sql_out = re.sub(r'\S+(?:\s+\|\|\s+\S+)+', lambda x: to_concat(x.group()), sql)
print(sql + "\n" + sql_out)

This prints:

select var1 || literal || var2 from yourTable
select concat(var1, literal, var2) from yourTable

The idea here is to first match the entire expression involving the ANSI || concatenation operator. Then, we pass this to a callback function which then selectively replaces all || by comma, as well as forms the function call to concat .

Answer 2

With the python re module, you can replace by regex by putting terms in parentheses in the pattern, then replacing then using \1 , \2 etc. in order of the terms.

re.sub(r'\s{1}([a-zA-Z0-9_]+)\s*\|\|\s*([a-zA-Z0-9_]+)\s*', r'concat \1 , \2', input_string)