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Understanding negative lookahead


Understanding negative lookahead

By : 이동훈
Date : November 15 2020, 06:54 AM
around this issue Lookaheads do not consume any characters. It just checks if the lookahead can be matched or not:
code :
a(?!b)c
ac
|
a

ac
 |
(?!b) #checks but does not consume. Pointer remains at c

ac
 |
 c
abc123
|
a

abc123
 |
 b

abc123
  c

abc123 #Tries to match 123; since is successful, the pointer remains at c
    |
 (?=123)

abc123 # Match is success. Further matching of patterns (if any) would proceed from this position
  |

abc123
   |
  \d

abc123
    |
   \d

abc123 #Reaches the end of input. The pattern is matched completely. Returns a successfull match by the regex engine
     |
    \d


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Negative lookahead vs Positive lookahead syntax

Negative lookahead vs Positive lookahead syntax


By : Kotteswaran Balaiyan
Date : March 29 2020, 07:55 AM
will be helpful for those in need The negative lookahead need to be nested within multiple sets of parentheses with the qualifier . and the quantifier * is called a tempered greedy token. You do not have to use it in this scenario.
You can use a normal lookahead anchored at the start instead of the tempered greedy token:
code :
^(?!.*foo).*$
Understanding why negative lookahead is not working

Understanding why negative lookahead is not working


By : Steaven Chédé
Date : March 29 2020, 07:55 AM
I wish this help you Let's say I have this url: , You could use:
code :
^                         # anchor it to the beginning
https?://                 # http or https
(?:
    (?!tbm=(?:isch|news)) # first neg. lookahead
    (?!param1=432)        # second
    \S                    # anything but whitespace
)+
$                         # THE END
Regex with balancing groups works with negative lookahead but not positive lookahead (.net dialect)

Regex with balancing groups works with negative lookahead but not positive lookahead (.net dialect)


By : Orin A
Date : March 29 2020, 07:55 AM
will be helpful for those in need You do not need the positive lookahead, use capturing and stack with conditional check:
code :
layer\s*{(?<f>\s*foo)?(?>{\s*foo(?<f>)(?<c>)|{(?<c>)|[^{}]+|}(?<-c>))*(?(c)(?!))(?(f)|(?!))}
Combining Positive Lookahead and Negative Lookahead - Regex

Combining Positive Lookahead and Negative Lookahead - Regex


By : Metik
Date : March 29 2020, 07:55 AM
To fix this issue I'm writing a simple regex in postgres 9.5 using ~ , Here is a solution which only uses a single negative lookahead:
code :
SELECT *
FROM yourTable
WHERE col ~ 'CHANGE.+SHOES(?!.*NIKE)';
Why positive lookahead is working but negative lookahead doesn't?

Why positive lookahead is working but negative lookahead doesn't?


By : user3266544
Date : March 29 2020, 07:55 AM
seems to work fine First of all, regex needs to be working for both the python and PCRE(PHP). I'm trying to ignore if a regex pattern is followed by the letter 'x' to distinguish dimensions from strings like "number/number" in the given example below: , You may use this simplified regex with negative lookahead:
code :
((\d*(?:,?\.?)\d*(?:,?\.?))\s?\/\s?(\d*(?:,?\.?)\d*(?:,?\.?)))\b(?![.,]?\d|\s?x)
(\d+(?:[,.]\d+)*)\s*\/\s*(\d+(?:[,.]\d+)*)\b(?![.,]?\d|\s?x)
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