Replace pattern in Regex using variables and strings

Question

I am having trouble doing the following search and replace:

// Consider this string - Note that it may be much more complicated, with many more matches
string output = "\"$type\": \"SomeType\",";

// The variable to search and replace - In this case is set to "SomeType"
string myType = "SomeType";

// The non-working regex
output = Regex.Replace(output, @"\"\$type\"\:\s\"" + myType + @"\",", "NewType");

I would expect the following output:

"$type": "NewType",

instead of:

"$type": "SomeType",

I think there are 2 problems here, but I cannot figure out the syntax. One is the use of the string "type", the other one is that I am not using capture groups so that only myType gets replaced by "NewType" in the output string.

Wiktor Stribiżew · Accepted Answer · 2021-03-11 10:36:19Z

1

You want to use

output = Regex.Replace(output, $@"(?<=""\$type"":\s""){Regex.Escape(myType)}(?="",)", "NewType");

See the C# demo.

NOTES:

" must be escaped with another " in a verbatim string literal
Escape the variables inside regex patterns
Use lookarounds to wrap the pattern parts you need to keep after substitution.

The resulting regex looks like (?<="\$type":\s")NewType(?=",) and matches

(?<="\$type":\s") - a location that is immediately preceded with "$type": a whitespace and "
NewType - some text
(?=",) - a location that is immediately followed with ",.

answered Mar 11, 2021 at 10:36

Wiktor Stribiżew

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4 Comments

stackMeUp Over a year ago

Ouha, much trickier than I thought, thanks a lot!

stackMeUp Over a year ago

What if myType was defined in the following line of the string, but had to be inserted as previously done? Can I do something of the sort: Regex.Replace(g, $@"(?<=""\$type"":\s""){"AString"}("",\s*\n\s*""Name"":\s""{myVariable}?="",)", myVariable, RegexOptions.IgnoreCase);

Wiktor Stribiżew Over a year ago

@stackMeUp You may use any amount of variables in the regex pattern.

stackMeUp Over a year ago

Thank, I figured it out :-)

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