How to search and add a string before the pattern in python using regular expressions

Question

Iam writing a program to find and replace a string from all the files with the given extension.
Here Iam using regular expressions for searching.The task is to findall the occurances and modify those.

If my string is "The number is 1234567890"
Result after searching and replacing should be +911234567890

I think i can re.sub() here like

s = "The number is 1234567890"
re.sub(r"\d{10}",??,s)

What can be given as the second argument here i don't know what the number would be i have modify the same matched string by preceding it with +91
I could do it using the findall from re and replace from string like

s = "The number is 1234567890 and 2345678901"
matches = re.findall(r'\d{10}',s)
for match in matches:
    s = s.replace(match,"+91"+match)

After this s is The number is +911234567890 and +912345678901

Is this the only way of doing it??Is it not possible using re.sub() ?? If it is please help me.Thank you...!

Gurmanjot Singh · Accepted Answer · 2018-03-29 03:17:59Z

0

Try this regex:

(?=\d{10})

Click for Demo

Explanation:

(?=\d{10}) - positive lookahead to find a zero-length match which is immediately followed by 10 digits

Code

import re
regex = r"(?=\d{10})"
test_str = "The number is 1234567890"
subst = "+91"
result = re.sub(regex, subst, test_str, 0)
if result:
    print (result)

Code Output

OR

If I use your regex, the code would look like:

import re
regex = r"(\d{10})"
test_str = "The number is 1234567890"
subst = "+91$1"
result = re.sub(regex, subst, test_str, 0)
if result:
    print (result)

edited Mar 29, 2018 at 3:17

answered Mar 29, 2018 at 3:08

Gurmanjot Singh

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5 Comments

MOHAMMED NUMAN Over a year ago

What does $1 do??

Gurmanjot Singh Over a year ago

$1 contains the contents of Group 1 which, in this case, is the 10 digit number.

MOHAMMED NUMAN Over a year ago

Can you tell me resource for learning about positive lookahead.

Gurmanjot Singh Over a year ago

THIS is a good website to begin with

MOHAMMED NUMAN Over a year ago

Thank you for helping :)

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