Regex match a substring if that substring is not preceded by a specific string?

问题

I thought this would be a common and easy thing to do, but so far I've been unable to find an answer by Googling.

How would I regex match a substring only if it is not immediately preceded by a specific string?

In this case I want to match " (a space followed by a quotation mark) but only if it's not preceded by , (a comma). I've tried a few different things. Currently (?!,) " is as close as I've gotten, but the negative lookahead doesn't seem to be working as I expect.

Some examples of what I want:

Laura said "What have you done?"            - 1 match
Laura said, "What have you done?"           - 0 matches (there's a comma)
"Come with me," David said.                 - 0 matches (no space before the ")
Michael said "Throw it!" Jessica replied, "I'd rather not..." Shia shouted "Just do it!"
- 2 matches

This is in Notepad++. What am I doing wrong?

回答1:

This seemed to work for me, tested in Notepad++:

[^,]\K "

回答2:

You can use double assertions to apply two conditions.

(?<= )(?<!, )"

(?<= ) -> Positive lookbehind assertion which asserts that the character going to be matched must be preceded by a space character.

(?<!, ) -> Negative lookbehind which asserts that the character going to be matched must not be preceded by a ,<space> characters.

" -> match the double quotes only if both conditions gets satisfied.

(?<=[^,] )"|(?<=^ )"

(?<=[^,] )" -> positive lookbehind which asserts that the double quotes must be preceeded by

any char but not of , [^,]
followed by a space character

And note that this regex won't match a double quotes preceded by a space char exists at the line start like this *. In-order to match the quotes exists at the line start, we have to use another condition like

(?<=^ )" matches the quotes which was preceded by,

^ start of the line.
<space> character

On ORing these two conditions with | OR operator, you can achieve the expected results.

DEMO

If you want to match also the preceding space then remove the space from lookahead assertions.

(?<!,) "

(?<!,) asserts that the chars going to be matched won't be preceded by ,.

DEMO

来源：https://stackoverflow.com/questions/48071699/regex-match-a-substring-if-that-substring-is-not-preceded-by-a-specific-string

标签

regex

notepad++