Using re.sub() to replace some part of a string

Question

background

I'm trying to use re to replace some value in a string. Essentially, replace the first detected int (in a specific pattern ) with 1 .

Examples:

{1}        -> {1}
{2}        -> {1}
{3,}       -> {1,}
{3,16}     -> {1,16}
ab{3,16}   -> ab{1,16}
5a4b{3,16} -> 5a4b{1,16}
{1,4}      -> {1,4}

attempt

import re

pattern = '.*{(\d+)}|{(\d+),}|{(\d+),\d+}'
string = 'ab{3,16}'
print re.sub(pattern, '1', string)

But I get ab1 instead of ab{1,16}

Answer 1

You don't want to replace the {} or , , so you could use positive lookaheads and lookbehinds in order to make sure the int is :

• after a { with : (?<={)
• before a } or a , with : (?=}|,)

If you want to want to replace only the first occurence in the whole string, you can set the 4th argument of sub to 1. In that case, {3}{3} will be converted to {1}{3} . UPDATE : You don't want it. :)

import re

pattern = '(?<={)\d+(?=}|,)'
string = 'ab{3,16}'
print re.sub(pattern, '1', string)

Here are some tests :

import re

def replace_first_int(string):
  pattern = '(?<={)\d+(?=}|,)'
  return re.sub(pattern, '1', string)

print replace_first_int('{1}')
# {1}
print replace_first_int('{2}')
# {1}
print replace_first_int('{3,}')
# {1,}
print replace_first_int('{3')
# {3
print replace_first_int('{3,16}')
# {1,16}
print replace_first_int('{1,4}')
# {1,4}
print replace_first_int('{3}{3}')
# {1}{1}
print replace_first_int('ab5')
# ab5
print replace_first_int('5a4b{3,16}')
# 5a4b{1,16}

Answer 2

When doing a replace with regex the whole matched string will be replaced. As you match the whole {1,16} , it is replaced by 1 accordingly.

For your samples (?<=\\{)\\d+ will work as pattern and can then replaced by 1 , like

import re

pattern = '(?<={)\d+'
string = 'ab{3,16}'
print re.sub(pattern, '1', string)

Answer 3

如果您的模式始终采用指定的格式（如您的示例中所示），那么就足够了：

print re.sub(r'{\d+', '{1', string, count=1)