Javascript正则表达式负提前匹配

Question

I suck at regex. I have the following expression:

(?!.*\\")(Level)(\\s)([0-9])(?!.*\\")

I want to use this expression to replace all Level 2 to "Level 2" with queries. The problem is that this regex catches "Level 2 (without closing " ). I want to catch Level 2 only if it has to quotes before AND after the words.

What is the best approach to do that?

Answer 1

If you just want to capture every occurrence of Level\\s+\\d+ and replace it with "Level\\s+\\d+" provided it is not enclosed by doublequotes from both sides, you may use try using this regex,

([^"]|^)(Level\s+\d+)([^"]|$)

and replace it with,

\1"\2"\3

Check here for a demo

This will not choose a text for replacement of type Label 2" or "Label 2 or "Label 2" and only replace when Label 2 when it is not surrounded by double quote from either side.

Answer 2

You may use

 var s = '"Level 3 protected": Level 4 here and "another Level 5"'; s = s.replace(/(^|[^"])(Level\\s+\\d+)(?!["\\d])/g, '$1"$2"'); console.log(s); 

Details

• (^|[^"]) - Group 1 ( $1 ): start of string or any char but a double quote
• (Level\\s+\\d+) - Group 2 ( $2 ): Level , 1+ whitespaces, 1+ digits
• (?!["\\d]) - no digit or double quote to the left of the current location is allowed.

See the regex demo .

Answer 3

^{Not sure if you want to replace "Level 2 and Level 2" or not, so I offer solutions for both scenarios:}

1) If you want to replace only those with not even a single " :

(?<!")(Level\\s\\d+)(?!")

see https://regex101.com/r/Nc7b5h/1

This uses a negative look-behind and a negative look-ahead

2) However, if you also want to replace those with only one " , try this:

(?!"Level\\s\\d+")(?<!")"?Level\\s\\d+"?

https://regex101.com/r/qL5Li5/1/

Set your match group based on whether you want to replace the single " or not:

var s='This "Level 2" is "Level 2 a Level 2" test Level 2 :)'
​
s.replace(/(?!"Level\s\d+")(?<!")"?(Level\s\d+)"?/g, '"$1"');

//> 'This "Level 2" is "Level 2" a "Level 2" test "Level 2" :)'

or

var s='This "Level 2" is "Level 2 a Level 2" test Level 2 :)'

s.replace(/(?!"Level\s\d+")(?<!")("?Level\s\d+"?)/g, '"$1"');

//> 'This "Level 2" is ""Level 2" a "Level 2"" test "Level 2" :)'

The trick here was to first perform a negative look-ahead for the forbidden case of a match with two " , before doing anything else.

General Notes

If you don't want to use a look-behind expression ( (?<!") ) as older browsers (pre-ES2018) won't support that, just use (^|[^"]) instead, as other suggested. Just don't forget to restore its contents in the replacement :)

eg s.replace(/(?!"Level\\s\\d+")(^|[^"])"?(Level\\s\\d+)"?/g, '$1"$2"');