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sub() method will replace all pattern occurrences in the target string. By setting the count=1 inside a re. sub() we can replace only the first occurrence of a pattern in the target string with another string. Set the count value to the number of replacements you want to perform.
If you want to replace a string that matches a regular expression (regex) instead of perfect match, use the sub() of the re module. In re. sub() , specify a regex pattern in the first argument, a new string in the second, and a string to be processed in the third.
sub() function belongs to the Regular Expressions ( re ) module in Python. It returns a string where all matching occurrences of the specified pattern are replaced by the replace string. To use this function, we need to import the re module first.
re. sub() function is used to replace occurrences of a particular sub-string with another sub-string. This function takes as input the following: The sub-string to replace.
The answer is:
re.sub(r'(foo)', r'\g<1>123', 'foobar')
Relevant excerpt from the docs:
In addition to character escapes and backreferences as described above, \g will use the substring matched by the group named name, as defined by the (?P...) syntax. \g uses the corresponding group number; \g<2> is therefore equivalent to \2, but isn’t ambiguous in a replacement such as \g<2>0. \20 would be interpreted as a reference to group 20, not a reference to group 2 followed by the literal character '0'. The backreference \g<0> substitutes in the entire substring matched by the RE.
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