G. Ünther G. Ünther - 3 years ago 158
Vb.net Question

VB/Regex: Replace character with conditions on preceding and following characters

I want to turn any zero that is not preceded by a space and zero or arbitrary many numbers and not followed by zero or an arbitrary amount of numbers and a + or - into a dot.

So far, I've managed to replace every zero not followed by zero or an arbitrary amount of numbers and a + or - with a dot, using this:

Str1 = " 0 0c000c0+c0-c 10c0c"
Str2 = Regex.Replace(Str1, "(?!\s+\d*)0(?!\d*[\+\-])", ".")


This returns " . .c...c0+c0-c 1.c.c", so the last part is working, the first one isn't.

I want that zeros are replaced if the are following this pattern:

<stuff><not a space followed by numbers>0<other numbers not followed by a plus or minus><more stuff>


Examples:

" 100c" >>> " 100c"
"c00c10+" >>> "c..c10+"
" 0 0cc0c0-" >>> " 0 0cc.c0-"


How can I get this to work?

Answer Source

You need to use a lookbehind, not a lookahead. Use

(?<!\s\d*)0(?!\d*[+-])
^^^^^^^^^

See the regex demo.

There is no need to use + after \s in the lookbehind, as a single whitespace check is enough.

Note there is no need to escape + in the character class and no need to escape - if it is at the end/start of the character class.

Details

  • (?<!\s\d*) - a negative lookbehind that fails the match if there is a whitespace and 0+ digits immediately to the left of the current location
  • 0 - a zero
  • (?!\d*[+-]) - a negative lookahead that fails the match if there are 0+ digits followed with - or + immediately to the right of the current location.
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