匹配字符串的python regex子部分

Question

I want to substitute 1st part of the regex matched string. I am using re (regex) package in python for this:

import re
string = 'H12-H121'
sub_string = 'H12'
re.sub(sub_string,"G12",string)

>> G12-G121

Expected Output:

>> G12-H121

Answer 1

You should tell engine that you want to do matching and substitution to be done at beginning using ^ anchor:

re.sub('^H12', 'G12', string)

or if you are not sure about string after - :

re.sub('^[^-]+', 'G12', string)

Live demo

If you only need to replace first occurrence of H12 use parameter count :

re.sub('H12', 'G12', string, count = 1)

^[^-]+ breakdown:

• ^ Match start of input string
• [^-]+ Match one or more character(s) except -

Answer 2

add a 1 for the count of occurrences to replace to the call to re.sub. what i mean is:

import re
string = 'H12-H121'
sub_string = 'H12'
re.sub(sub_string,"G12",string, 1)  #<---- 1 added here

now the output is 'G12-H121' since it only replaces the first match

Answer 3

You can do this with just a str.replace()

full_string = 'H12-H121'
sub_string = 'H12'
output_string = full_string.replace(sub_string,"G12",1)

print(output_string)

outputs:

G12-H121

Answer 4

Just add a ^ to the substring re pattern

import re

s1 = 'H12-H121'
pat = r'^H12'
print(re.sub(pat,"G12",s1))

outputs G12-H121