Prepend a substring if the source string does not begin with the same substring

Question

I’m trying to create a valid RegEx replacement pattern for correctly formatting specific .XML file names. Specifically, if a file name does not begin with a prefix, let’s say, ACE_. If the XML file name and extension does not begin with ACE_, prepend the string ACE_ to the file name. For example, if my input source string is the following:

Widgets.xml

I would like to execute a single RegEx Replacement that would result in the string being:

ACE_Widgets.xml.

Conversely, if the string already begins with ACE_, I would like it to remain unchanged.

Also, how can I include the pattern “.xml” to ensure that the string pattern for the file name and extension ends with “.xml” in the same matching pattern for the RegEx Replacement pattern?

As for the match, I have some luck with the following:

^ACE_{1}[\d\D]+

Which indicates there is a match for the pattern if the input string is ACE_Widgets.xml and no match if the string is Widgets.xml

The RegEx pattern would suffice, but if you need to know the language in which I’d like to use the replacement pattern is in .NET 4.0 in either C# or VB.NET.

The following posting is close to what I’m looking for, but with the inclusion of the *ix directory path prefix, and the use of preg_replace() in PHP, I’m having a bit of a struggle getting it to work with what I need to do:

Regular Expression: How to replace a string that does NOT start with something?

As always, thank you in advance for your time, help and patience. They are very much appreciated…

Answer 1

Actually regex is a pretty clean way to accomplish this. Here is a solution with a regex written in free-spacing mode with lots-o comments:

text = Regex.Replace(text, @"
    # Match XML file names not starting with: ACE_
    ^          # Anchor to start of string.
    (?!ACE_)   # Assert does not start with ACE_
    .*         # Match filename up to extension.
    (?i)       # Make case insensitive for .xml
    \.xml      # Require XML file extension.
    $          # Anchor to end of string.", 
    "ACE_$0", RegexOptions.IgnorePatternWhitespace);

Note that this assumes that the ACE_ prefix requirement is case sensitive.

Answer 2

There is no need for regex here:

var fileName = …;
var prefix = "ACE_"
if (!fileName.StartsWith(prefix))
    fileName = prefix + fileName;

If you want to add checking for the .xml extension too, add:

var extension = ".xml";
if (!fileName.EndsWith(extension))
    fileName = fileName + extension;