如何使用python中的正则表达式替换未包含在标记中的子字符串

Question

I have sentences.

text="The president of America is <PERSON>Barack Obama</PERSON>. He was born on August 4, 1961. Obama was reelected president in November 2012".

I want to put <PERSON></PERSON> tag in "Obama", so the result will be like this:
The president of America is <PERSON>Barack Obama</PERSON>. He was born on August 4, 1961. <PERSON>Obama</PERSON> was reelected president in November 2012".

I want to find substring(example: Obama) that there is no tag <PERSON> before the substring and there is no tag </PERSON> after the substring, but I don't know the right syntax for regex in python.
**I'm new to python :''

With simple regex re.sub(namedEntity, "<PERSON>"+namedEntity+"</PERSON>", text) will give an output
The president of America is <PERSON>Barack <PERSON>Obama</PERSON></PERSON>. He was born on August 4, 1961. <PERSON>Obama</PERSON> was reelected president in November 2012".

this is my code(using python2.7)

import re

result=re.sub(r"((?!<PERSON>).*"+namedEntity+".*(?!</PERSON>))","<PERSON>"+namedEntity+"</PERSON>",text)

print "result: "+result

The output
result: <PERSON>Obama</PERSON>
And I don't know that is the first "Obama" or the second one.

Thanks for your help before

Answer 1

You are very close. In your new regex r"((?!<PERSON>).*"+namedEntity+".*(?!</PERSON>))" , you have .* before and after which matches 'Obama' with any characters before and after it and the lookarounds are ignored because the tags are in the matched group. If you remove them, you get the results you're after.

>>> import re
>>> text = "The president of America is <PERSON>Barack Obama</PERSON>. He was born on August 4, 1961. Obama was reelected president in November 2012"
>>> namedEntity = 'Obama'
>>> result = re.sub(r"((?!<PERSON>)"+namedEntity+"(?!</PERSON>))","<PERSON>"+namedEntity+"</PERSON>",text)
>>> print result
'The president of America is <PERSON>Barack Obama</PERSON>. He was born on August 4, 1961. <PERSON>Obama</PERSON> was reelected president in November 2012'

For future regex testing, regex101 works well to check how things work as you change them live. For your case this shows what's happening.

Answer 2

just remove the .* part in your regex-lookarounds.

>>>text="The president of America is <PERSON>Barack Obama</PERSON>. He was born on August 4, 1961. Obama was reelected president in November 2012"
>>> surname=re.search(r'<PERSON>(.*)</PERSON>', text).group(1).split()[1]
>>> print surname
Obama
>>> re.sub(r'(?<!<PERSON>)'+surname+'(?!</PERSON>)', '<PERSON>'+surname+'</PERSON>', text)'  
The president of America is <PERSON>Barack Obama</PERSON>. He was born on August 4, 1961. <PERSON>Obama</PERSON> was reelected president in November 2012'
>>> 

Note: you can also extract the surname of the person using regex and capture groups which i have captured in surname variable. You can use (?<!regex) to assert negative lookbehind and (?!regex) to assert negative lookahead