Regular expression to convert given number in the required format

Question

I am first time using regular expression hence need help with one slightly complex regular expression. I have input list of around 100-150 string object(numbers).

input = ['90-10-07457', '000480087800784', '001-713-0926', '12-710-8197', '1-345-1715', '9-23-4532', '000200007100272']

Expected output = ['00090-00010-07457', '000480087800784', '00001-00713-00926', '00012-00710-08197', '00001-00345-01715', '00009-00023-04532', '000200007100272']

## I have tried this -

import re
new_list = []
for i in range (0, len(input)):
    new_list.append(re.sub('\d+-\d+-\d+','0000\\1', input[i]))

## problem is with second argument '0000\\1'. I know its wrong but unable to solve
print(new_list)  ## new_list is the expected output.

As you can see, I need to convert string of numbers coming in different formats into 15 digit numbers by adding leading zeros to them.

But there is catch here i.e. some numbers i.e.'000480087800784' are already 15 digits, so should be left unchanged (That's why I cannot use string formatting (.format) option of python) Regex has to be used here, which will modify only required numbers. I have already tried following answers but not been able to solve.

I suspect a [regex way](https://ideone.com/jr2CIP) is not the best here. — Wiktor Stribiżew, Jan 17 '19 at 11:55
Hi @WiktorStribiżew I tried your solution but it did not work. Can you plz explain how should I use \1 — Sanchit Joshi, Jan 17 '19 at 11:56
Do not use `\1` as your regex has no capturing group. I think you did not try my solution if it does not work. Use https://ideone.com/AGlQLJ — Wiktor Stribiżew, Jan 17 '19 at 11:58
Thanks a ton @WiktorStribiżew I am using python for last 4 month but never had to use regular expression hence I was struggling with it. Thanks for the help..! — Sanchit Joshi, Jan 17 '19 at 12:00
Hi @WiktorStribiżew Yes it worked. If you can post the second solution here, I will mark it as accepted answer..! and can you plz remove duplicate mark bcz it was not really same question. Thanks again. — Sanchit Joshi, Jan 17 '19 at 12:44
I posted [the answer some time ago](https://stackoverflow.com/a/54235592/3832970). — Wiktor Stribiżew, Jan 17 '19 at 12:44

score 1 · Accepted Answer · answered Jan 17 '19 at 12:07

Your regex does not work as you used \1 in the replacement, but the regex pattern has no corresponding capturing group. \1 refers to the first capturing group in the pattern.

If you want to try your hand at regex, you may use

re.sub(r'^(\d+)-(\d+)-(\d+)$', lambda x: "{}-{}-{}".format(x.group(1).zfill(5), x.group(2).zfill(5), x.group(3).zfill(5)), input[i])

See the Python demo.

Here, ^(\d+)-(\d+)-(\d+)$ matches a string that starts with 1+ digits, then has -, then 1+ digits, - and again 1+ digits followed by the end of string. There are three capturing groups whose values can be referred to with \1, \2 and \3 backreferences from the replacement pattern. However, since we need to apply .zfill(5) on each captured text, a lambda expression is used as the replacement argument, and the captures are accessed via the match data object group() method.

However, if your strings are already in correct format, you may just split the strings and format as necessary:

for i in range (0, len(input)):
    splits = input[i].split('-')
    if len(splits) == 1:
        new_list.append(input[i])
    else:
        new_list.append("{}-{}-{}".format(splits[0].zfill(5), splits[1].zfill(5), splits[2].zfill(5)))

See another Python demo. Both solutions yield

['00090-00010-07457', '000480087800784', '00001-00713-00926', '00012-00710-08197', '00001-00345-01715', '00009-00023-04532', '000200007100272']

score 0 · Answer 2 · answered Jan 17 '19 at 11:53

How about analysing the string for numbers and dashes, then adding leading zeros?

input = ['90-10-07457', '000480087800784', '001-713-0926', '12-710-8197', '1-345-1715', '9-23-4532', '000200007100272']
output = []
for inp in input:
    # calculate length of string
    inpLen = len(inp)
    # calculate num of dashes
    inpDashes = inp.count('-')
    # add specific number of leading zeros
    zeros = "0" * (15-(inpLen-inpDashes))
    output.append(zeros + inp)
print (output)

>>> ['00000090-10-07457', '000480087800784', '00000001-713-0926', '00000012-710-8197', '00000001-345-1715', '000000009-23-4532', '000200007100272']

Hi @Asmox Appreciate your help, but its not exactly what I am looking for. But, still your approach is really interesting. Thanks — Sanchit Joshi, Jan 17 '19 at 13:25

Regular expression to convert given number in the required format

2 Answers2