Question

I need to find attribute values in an ASPX file using regular expressions.

That means you don’t need to worry about malformed HTML or any HTML related issues.

I need to find the value of a particular attribute (LocText). I want to get what’s inside the quotes.
Any ASPX tags such as <%=, <%#, <%$ etc. inside the value don’t make sense for this attribute therefore are considered as part of it.

The regex I began with looks like this:

LocText="([^"]+)"

This works great, the first group, which is the result text, gets everything except the double quotes, which are not allowed there (&quot ; must be used instead)

But the ASPX file allows using of single quotes – second regular expression must be applied then.

LocText='([^']+)'

I could use these two regular expressions but I’m looking for a way to connect them.

LocText=("([^"]+)"|'([^']+)')

This also works but doesn’t seem very efficient as it’s creating unnecessary number of groups. I think this could be somehow done by using backreferences, but I can’t get it to work.

LocText=(["']{1})([^\1]+)\1

I thought that by this, I save the single/double quote to the first group and then I tell it to read anything that is NOT the char found in the first group. This is enclosed again by the quote from the first group. Obviously, I’m wrong and it’s not working like that.

Is there any way, how to connect the first two expressions together creating just a minimum amount of groups with one group being the value of the attribute I want to get? Is it possible using a backreference for the single/double quote value, or have I completely misunderstood the meaning of them?

Answer 1

I’d say your solution with alternation isn’t that bad, but you could use named captures so the result will always be found in the same group’s value:

Regex regexObj = new Regex(@"LocText=(?:""(?<attr>[^""]+)""|'(?<attr>[^']+)')");
resultString = regexObj.Match(subjectString).Groups["attr"].Value;

Explanation:

LocText=          # Match LocText=
(?:               # Either match
 "(?<attr>[^"]+)" # "...", capture in named group <attr>
|                 # or match
 '(?<attr>[^']+)' # '...', also capture in named group <attr>
)                 # End of alternation

Another option would be to use lookahead assertions ([^\1] isn’t working because you can’t place backreferences inside a character class, but you can use them in lookarounds):

Regex regexObj = new Regex(@"LocText=([""'])((?:(?!\1).)*)\1");
resultString = regexObj.Match(subjectString).Groups[2].Value;

Explanation:

LocText=   # Match LocText=
(["'])     # Match and capture (group 1) " or '
(          # Match and capture (group 2)...
 (?:       # Try to match...
  (?!\1)   # (unless it's the quote character we matched before)
  .        # any character
 )*        # repeat any number of times
)          # End of capturing group 2
\1         # Match the previous quote character