如何通过正则表达式用保存部分模式替换字符串的一部分？

Question

For example, I have strings like this:

string s = "chapter1 in chapters"

How can I replace it with regex to this:

s = "chapter 1 in chapters"

eg I need only to insert whitespace between "chapter" and it's number if it exists. re.sub(r'chapter\\d+', r'chapter \\d+ , s) doesn't work.

Answer 1

You can use lookarounds:

>>> s = "chapter1 in chapters"
>>> print re.sub(r"(?<=\bchapter)(?=\d)", ' ', s)
chapter 1 in chapters

RegEx Breakup:

(?<=\bchapter)  # asserts a position where preceding text is chapter
(?=d)           # asserts a position where next char is a digit

Answer 2

You can use capture groups, Something like this -

>>> s = "chapter1 in chapters"
>>> re.sub(r'chapter(\d+)',r'chapter \1',s)
'chapter 1 in chapters'