Support accent characters in cite names.

A new commit introduces a cite name that contains an accent character:
@inproceedings{Müller2024a,

It will cause the bibtex parser to fail with the following two errors:

failed to extract cite name of: @inproceedings{Müller2024a,

and

parse failed at 55:17: syntax error: unexpected $end, expecting tCOMM

The second error is an upstream limitation,
which I will try to get it merged to the upstream: https://github.com/nickng/bibtex.

src/main.go

@@ -13,8 +13,9 @@ import (
 	"github.com/nickng/bibtex"
 )
 
-// Matches e.g.: @inproceedings{Doe2024a,
+// Matches e.g.: @inproceedings{Müller2024a,
-var re = regexp.MustCompile(`@[a-z]*\{([A-Za-z\-]*[0-9]{4}[a-z]),`)
+// \p{L}\p{M} matches any letter, including accented characters.
+var re = regexp.MustCompile(`@[a-z]*\{([\"\p{L}\p{M}\-]*[0-9]{4}[a-z]),`)
 
 // Map a cite name (e.g., Doe2024a) to its line number in the .bib file. All
 // cite names are unique.

src/vendor/github.com/nickng/bibtex/token.go

@@ -29,8 +29,18 @@ func isWhitespace(ch rune) bool {
 	return ch == ' ' || ch == '\t' || ch == '\n' || ch == '\r'
 }
 
+func isAccent(ch rune) bool {
+	accents := "äöüßéêçñÁÉÍÓÚáéíóúàèìòùâêîôûãõñÄÖÜ"
+	for _, accent := range accents {
+		if ch == accent {
+			return true
+		}
+	}
+	return false
+}
+
 func isAlpha(ch rune) bool {
-	return ('a' <= ch && ch <= 'z') || ('A' <= ch && ch <= 'Z')
+	return ('a' <= ch && ch <= 'z') || ('A' <= ch && ch <= 'Z') || isAccent(ch)
 }
 
 func isDigit(ch rune) bool {