Can Regex be used for this particular string manipulation?

Question

I need to replace character (say) x with character (say) P in a string, but only if it is contained in a quoted substring. An example makes it clearer:

axbx'cxdxe'fxgh'ixj'k  -> axbx'cPdPe'fxgh'iPj'k

Let's assume, for the sake of simplicity, that quotes always come in pairs.

The obvious way is to just process the string one character at a time (a simple state machine approach);
however, I'm wondering if regular expressions can be used to do all the processing in one go.

My target language is C#, but I guess my question pertains to any language having builtin or library support for regular expressions.

Answer 1

I converted Greg Hewgill's python code to C# and it worked!

[Test]
public void ReplaceTextInQuotes()
{
  Assert.AreEqual("axbx'cPdPe'fxgh'iPj'k", 
    Regex.Replace("axbx'cxdxe'fxgh'ixj'k",
      @"x(?=[^']*'([^']|'[^']*')*$)", "P"));
}

That test passed.

Answer 2

I was able to do this with Python:

>>> import re
>>> re.sub(r"x(?=[^']*'([^']|'[^']*')*$)", "P", "axbx'cxdxe'fxgh'ixj'k")
"axbx'cPdPe'fxgh'iPj'k"

What this does is use the non-capturing match (?=...) to check that the character x is within a quoted string. It looks for some nonquote characters up to the next quote, then looks for a sequence of either single characters or quoted groups of characters, until the end of the string.

This relies on your assumption that the quotes are always balanced. This is also not very efficient.