Creating a valid string from a regular expression

Question

Hello,

In the application I am currently working on, I have an option to create automatic backups of a certain file on the hard disk. What I would like to do is offer the user the possibility to configure the name of the file and its extension.

For example, the backup filename could be something like : "backup_month_year_username.bak". I had the idea to save the format in the form of a regular expression. For the example above, the regexp would look like :

"^backup_(?<Month>\d{2})_(?<Year>\d{2})_(?<Username>\w).(?<extension>bak)$"

I thought about using regex because I will also have to browse through the directory of backuped files to delete those older than a certain date. The main trouble I have now is how to create a filename using the regex. In a way I should replace the tags with the information. I could do that using regex.replace and another regex, but I feel it's a big weird doing that and it might be a better way.

Thanks

[Edit] Maybe I wasn't really clear in the first go, but the idea is of course that the user (in this case an admin that will know regex syntax) will have the possibility to modify the form of the filename, that's all the idea behind it[/Edit]

Answer 1

If the filename is always in this form, there is no reason for a regex, as it's easier to process with string.Split ...

Answer 2

... and if the regex changes, it is next to impossible to reconstruct a string from a given regex.

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Edit:

Create some predefined "place-holders": %u could be the user's name, %y could be the year, etc.:

backup_%m_%y_%u.bak

and then simple replace the %? with their actual values.

Answer 3

With Bart's solution it is easy enough to split (using string.Split) the generated file name using underscore as the delimiter, to get back the information.

Answer 4

It sounds like you're trying to use the regular expression to create the file name from a pattern which the user should be able to specify.

Regular expressions can - AFAIK - not be used to create output, but only to validate input, so you'd have the user specify two things:

1. a file name production pattern like Bart suggested
2. a validation pattern in form of a regular expression that helps you split the file names into their parts

EDIT
By the way, your sample regex contains an error: The "." is use for "any character", also \w only matches one word character, so I guess you meant to write

"^backup_(?<Month>\d{2})_(?<Year>\d{2})_(?<Username>\w+)\.(?<extension>bak)$"

Answer 5

Ok, I think I have found a way to use only the regex. As I am using groups to get the information, I will use another regular expression to match the regular expression and replace the groups with the value:

Regex rgx = new Regex("\(\?\<Month\>.+?\)");
rgx.Replace("^backup_(?<Month>\d{2})_(?<Year>\d{2})_(?<Username>\w+)\.(?<extension>bak)$"
, DateTime.Now.Month.ToString());

Ok, it's really a hack, but at least it works and I have only one pattern defined by the user. It might not work if the regex is too complex, but I think I can deal with that problem.

What do you think?