将段落中句子的每个首字母大写

Question

I want to capitilize the first word after a dot in a whole paragraph (str) full of sentences. The problem is that all chars are lowercase.

I tried something like this:

text = "here a long. paragraph full of sentences. what in this case does not work. i am lost" 
re.sub(r'(\b\. )([a-zA-z])', r'\1' (r'\2').upper(), text) 

I expect something like this:

"Here a long. Paragraph full of sentences. What in this case does not work. I am lost."

Answer 1

You can use re.sub with a lambda :

import re
text = "here a long. paragraph full of sentences. what in this case does not work. i am lost" 
result = re.sub('(?<=^)\w|(?<=\.\s)\w', lambda x:x.group().upper(), text)

Output:

'Here a long. Paragraph full of sentences. What in this case does not work. I am lost'

Regex Explanation:

(?<=^)\\w : matches an alphanumeric character preceded by the start of the line.

(?<=\\.\\s)\\w : matches an alphanumeric character preceded by a period and a space.

Answer 2

You can use ((?:^|\\.\\s)\\s*)([az]) regex ( which doesn't depend upon lookarounds which sometimes may not be available in the regex dialect you may be using and hence is simpler and widely supported. Like for example Javascript doesn't yet widely support lookbehind although it is supported in EcmaScript2018 but its not widely supported yet ) where you capture either the starting zero or more whitespace at the beginning of a sentence or one or more whitespace followed by a literal dot . and capture it in group1 and next capture a lower case letter using ([az]) and capture in group2 and replace the matched text with group1 captured text and group2 captured letter by making it uppercase using lambda expression. Check this Python code,

import re

arr = ['here a long.   paragraph full of sentences. what in this case does not work. i am lost',
       '   this para contains more than one space after period and also has unneeded space at the start of string.   here a long.   paragraph full of sentences.  what in this case does not work. i am lost']

for s in arr:
    print(re.sub(r'(^\s*|\.\s+)([a-z])', lambda m: m.group(1) + m.group(2).upper(), s))

Output,

Here a long.   Paragraph full of sentences. What in this case does not work. I am lost
   This para contains more than one space after period and also has unneeded space at the start of string.   Here a long.   Paragraph full of sentences.  What in this case does not work. I am lost

And in case you want to get rid of extra whitespaces and reduce them to just one space, just take that \\s* out of group1 and use this regex ((?:^|\\.\\s))\\s*([az]) and with updated Python code,

import re

arr = ['here a long.   paragraph full of sentences. what in this case does not work. i am lost',
       '   this para contains more than one space after period and also has unneeded space at the start of string.   here a long.   paragraph full of sentences.  what in this case does not work. i am lost']

for s in arr:
    print(re.sub(r'((?:^|\.\s))\s*([a-z])', lambda m: m.group(1) + m.group(2).upper(), s))

You get following where extra whitespace is reduced to just one space, which may often be desired,

Here a long. Paragraph full of sentences. What in this case does not work. I am lost
This para contains more than one space after period and also has unneeded space at the start of string. Here a long. Paragraph full of sentences. What in this case does not work. I am lost

Also, if this was to be done using PCRE based regex engine, then you could have used \\U in the regex itself without having to use lambda functions and just been able to replace it with \\1\\U\\2

Regex Demo for PCRE based regex