re.sub in Python 3.3

Question

I am trying to change the text string from the form of file1 to file01. I am really new to python and can't figure out what should go in 'repl' location when trying to use a pattern. Can anyone give me a hand?

text = 'file1 file2 file3'

x = re.sub(r'file[1-9]',r'file\0\w',text) #I'm not sure what should go in repl.

Answer 1

You could try this:

>>> import re    
>>> text = 'file1 file2 file3'
>>> x = re.sub(r'file([1-9])',r'file0\1',text)
'file01 file02 file03'

The brackets wrapped around the [1-9] captures the match, and it is the first match. You will see I used it in the replace using \1 meaning the first catch in the match.

Also, if you don't want to add the zero for files with 2 digits or more, you could add [^\d] in the regexp:

x = re.sub(r'file([1-9](\s|$))',r'file0\1',text)

---

A bit more of a generic solution now that I'm revisiting this answer using str.format() and a lambda expression:

import re
fmt = '{:03d}'                 # Let's say we want 3 digits with leading zeroes
s = 'file1 file2 file3 text40'
result = re.sub(r"([A-Za-z_]+)([0-9]+)", \
                lambda x: x.group(1) + fmt.format(int(x.group(2))), \
                s)
print(result)
# 'file001 file002 file003 text040'

A bit of details about the lambda expression:

lambda x: x.group(1) + fmt.format(int(x.group(2)))
#         ^--------^   ^-^        ^-------------^
#          filename   format     file number ([0-9]+) converted to int
#        ([A-Za-z_]+)            so format() can work with our format

I am using the expression [A-Za-z_]+ assuming the filename contains letters and underscores only besides the training digits. Do pick a more appropriate expression if required.

Answer 2

To match files with single digit on the end, use a word boundary \b:

>>> text = ' '.join('file{}'.format(i) for i in range(12))
>>> text
'file0 file1 file2 file3 file4 file5 file6 file7 file8 file9 file10 file11'
>>> import re
>>> re.sub(r'file(\d)\b',r'file0\1',text)
'file00 file01 file02 file03 file04 file05 file06 file07 file08 file09 file10 file11'