Force match on filename*= over filename= in regular expression

Question

I have a Content-Disposition header as such:

Content-Disposition: attachment; filename="övrigt.xlsx"; filename*=utf-8''%C3%B6vrigt.xlsx

According to specs there is either a filename="filename.extension" and/or filename*=charencoding''filename.extension. When filename* is present it should be used over filename.

So I want to catch the filename and the character encoding in the filename* attribute over the filename attribute when present. I ended up with this regex:

filename\*?=(?:([^'"]*)''|("))([^;]+)\2(?:[;`\n]|$)

It works fine, the only problem I have is that it matches whatever comes first, filename* or filename:

1. attachment; filename*=utf-8''%C3%B6vrigt.xlsx; filename="övrigt.xlsx"

Matches:

Match 1
Full match  12-45   filename*=utf-8''%C3%B6vrigt.xlsx;
Group 1.    n/a     utf-8
Group 3.    n/a     %C3%B6vrigt.xlsx

2. attachment; filename="övrigt.xlsx"; filename*=utf-8''%C3%B6vrigt.xlsx

Matches:

Match 1
Full match  12-35   filename="övrigt.xlsx";
Group 2.    n/a     "
Group 3.    n/a     övrigt.xlsx

Group 1 always matches character encoding when present.
Group 3 always matches the filename.

So I can now use filename and decode when group1 is not empty...

---

So to get to the question:

As I understood the *? should greedily try to match filename with * (see reference here):

The question mark is the first metacharacter introduced by this tutorial that is greedy. The question mark gives the regex engine two choices: try to match the part the question mark applies to, or do not try to match it. The engine always tries to match that part. Only if this causes the entire regular expression to fail, will the engine try ignoring the part the question mark applies to.

Why does it not work as expected, what am I doing wrong. How can I achieve matching of filename*= over filename= if present.

Answer 1

Assuming there is only a single match expected, and the match is expected to be the last match occurrence, you can use

(?:.*filename\*|filename)=(?:([^'"]*)''|("))([^;]+)\2(?:[;`\n]|$)

See the regex demo.

The part I modified is the one before =, note that the part after = might also need adjusting, but this is not the point here.

The (?:.*filename\*|filename) non-capturing group contains two alternatives:

• .*filename\* - any zero or more chars other than line break chars, as many as possible, and then filename* substring
• | - or
• filename - just a filename substring.

Why it works:

• The regex engine starts parsing the string from left to right
• The non-capturing group pattern is triggered and the first alternative is tried
• .*filename\* will match if there is filename* anywhere to the right of the current location
• If there is no filename* the second alternative, filename, will be searched for at every location in the string, and once found, it will get matched. Else, there'll be no matches at all.