strange python regex: not able to find match-CodePudding

I am facing some strange python regex issue. The following two strings are supposedly to be exactly the same. But somehow they are not matching.

import re
print(" \\\"")
print(" " chr(92) chr(34) "")
print(re.search(" \\\"", " " chr(92) chr(34) ""))

However, the following does match

import re
print("\\\"")
print("" chr(92) chr(34) "")
print(re.search("\\\"", "" chr(92) chr(34) ""))

Any thought on what is going on here?

CodePudding user response：

Issue is the backslash character has special meaning to a string in python. You can use a Python raw string created by prefixing a string literal with 'r' or 'R' where python raw string treats backslash (\) as a literal character.

import re
print(" \\\"")
print(" " chr(92) chr(34) "")
print(re.search(r" \\\"", " " chr(92) chr(34) ""))

Output:

 \"
 \"
<re.Match object; span=(0, 3), match=' \\"'>

In second example print(re.search("\\\"", "" chr(92) chr(34) "")) outputs: <re.Match object; span=(1, 2), match='"'> where only the double quote is matched.

Need to escape the backslash or use a raw string. If use single-quotes around the regexp then the double-quote does not need to be escaped.

s = ""   chr(92)   chr(34)   ""
print(re.search("\\\\\"", s))
print(re.search(r"\\\"", s))
print(re.search(r'\\"', s))

Output:

<re.Match object; span=(0, 2), match='\\"'>
<re.Match object; span=(0, 2), match='\\"'>
<re.Match object; span=(0, 2), match='\\"'>

For further details of the raw string and backslash, see answers to this question.