Question

I have a string containing date string, ‘363898,Catullus,84-11-1 BC’
How can I replace year 84 with 0084 using Python re?

Answer 1

Try:

import re
re.sub(r',(\d\d)-', r',00\1-', '363898, Catullus,84-11-1 BC')

With r',(\d\d)-' we capture any pair of digits (not less or more) that is between a comma and a dash. We replace the whole match with a comma, two zeros, whatever was between in the first pair of parenthesis (here the two digits), and a dash.

It means:

• if there are no two digits between a coma and a dash, no replacement is made. You get back your original string
• if there are these digits, there are replace by themselves, surounded by the same comma and dash, but with additional zeros.

When trying regular expressions in Python, just play with an online regular expression tester Full disclaimer: I coded this one so I’m biased.

But maybe regular expressions are not the best fit for the job. Sometimes a good use of ordinary functions is just as good:

In [10]: entry = '363898,Catullus,84-11-1 BC'.split(',')
In [11]: numbers = entry[-1].split('-')
In [12]: numbers = '-'.join([numbers[0].zfill(4)] + numbers[1:])
In [13]: entry = entry = ','.join(entry[:-1] + [numbers])
In [14]: entry
Out[14]: '363898,Catullus,0084-11-1 BC'