Python正则表达式将第1组替换为某些字符串

Question

I wonder how to substitute group 1 with certain string by regex in python.

Question1:

str = "aaa bbb ccc"
regex = "\baaa (bbb)\b"
repl = "111 bbb 222"

Use regex to match str, matched "aaa bbb", and replace group1 "bbb" with "111 bbb 222", and get the result "aaa 111 bbb 222 ccc"

str_repl = "aaa 111 bbb 222 ccc"

Thanks for @RomanPerekhrest and @janos 's lookbehind method.

And I wonder how to solve a more general scenario:

Question2:

s1 = "bBb"
regex = "(?<=\baaa )" + s1 + "\b"  # may not suitable
repl = "XxX " + s1 + " YyY"

target:

s0 = "aaa bBb ccc"
s0_repl = "aaa XxX bBb YyY ccc"

s1 = "aaa bbb ccc"
no match

s2 = "AAA bBb ccc"
s2_repl = "AAA XxX bBb YyY ccc"

Ignore the case for substring except of s1 when matching in original string.

Question3:

s1 = "bbb"
regex = "(?<=\baaa )" + s1 + "\b"  # may not suitable
repl = "XxX " + s1 + " YyY"

target:

s0 = "aaa bBb ccc"
s0_repl = "aaa XxX bBb YyY ccc"

s1 = "aaa bbb ccc"
s1_repl = "aaa XxX bbb YyY ccc"

s2 = "AAA BBB ccc"
s2_repl = "AAA XxX BBB YyY ccc"

Ignore the case for substring except of s1 when matching & substituting in original string.

Question4:

If there is a way to substitute group 1 on original string by regex on python?

Answer 1

You can use the re package, and positive look-behind:

import re
s = "aaa bbb ccc"
regex = r"\b(?<=aaa )(bbb)\b"
repl = "111 bbb 222"
print(re.sub(regex, repl, s))

This will produce:

aaa 111 bbb 222 ccc

Notice the changes I did there:

• The aaa prefix in the regex is wrapped in (?<=...) . This means, match bbb if it follows aaa , without including aaa in the pattern to replace. This is called positive lookbehind . Without this change to your regex, the aaa would disappear together with bbb
• Regular expression strings should be written as r"..." , to make them raw strings, in order to avoid problems with escape sequences
• I renamed the str variable to s , because str is a reserved word in Python, as @elena also pointed out.

Answer 2

To replace sequence bbb which should be preceded by sequence aaa use the following approach:

s = "aaa bbb ccc"
regex = r"(?<=aaa )bbb\b"
repl = "111 bbb 222"

str_replaced = re.sub(regex, repl, s)
print(str_replaced)

The output:

aaa 111 bbb 222 ccc

---

(?<=aaa ) - lookbehind positive assertion, ensures that "bbb" is preceded by "aaa "

http://www.regular-expressions.info/lookaround.html

Answer 3

First of all, don't use str as a variable name. It's a reserved keyword in Python.

import re

str1 = "aaa bbb ccc"
re.sub("bbb", "111 bbb 222", str1)
Out[11]: 'aaa 111 bbb 222 ccc'