Regex matching the end of a line correctly

Question

I'm looking for a way to re.sub lines ending with \\n and \\r\\n while preserving the line ending.

import re
data = "Foo\r\nFoo\nFoo"
regex = r"^Foo$"
re.sub(regex, "Bar", data, flags=re.MULTILINE)

leaves me with

Foo\r\nBar\nBar

When using the regex ^Foo(\\r)?$ to check for an optional \\r , I'll end up with

Bar\nBar\nBar

Any ideas?

Edit : Expected result

Bar\r\nBar\nBar

Answer 1

Make the \\r and \\n optional with ? , and specify them as a capture group so you can refer to them in the callback function for the replacement:

>>> re.sub('^Foo(\r?\n?)$', r'Bar\1', 'Foo\r\nFoo\nFoo', flags=re.MULTILINE)
'Bar\r\nBar\nBar'

This will match lines ending in \\r , \\n , or \\r\\n , and lines without a linefeed at all.

Answer 2

Use positive lookahead assertion

import re
data = "Foo\r\nFoo\nFoo"
regex = r"^Foo(?=\r?\n?$)"
re.sub(regex, "Bar", data, flags=re.MULTILINE)

Output :

'Bar\r\nBar\nBar'

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Regex explanation here.