Character class for any letter, including all kinds of accents

The regular expression: /^[a-zA-ZÀ-ÖØ-öø-ÿ]+$/is composed of the following:

/ indica el inicio y fin de la expresión regular
^ inicio del string
$ fin del string   
[xyz] denota un conjunto de caracteres
a-z caracteres entre la a minuscula y la z minuscula charcode 97 al 
122
A-Z caracteres entre la A mayúscula y la Z mayúsucla charcode 65 al 90
À-Ö charcode 192 al 214
Ø-ö charcode 216 al 246
ø-ÿ charcode 248 al 255
+ una o mas caracteres

that is, the regular expression matches any string that has the characters between the square brackets [] and has a length of 1 or greater.

The regular expression uses as base the ascii characters https://www.ascii-code.com/

I also recommend you to use : https://regexr.com/ or https://regex101.com/ to evaluate your expressions

Complementing FelipeM's answer , the character class [a-zA-ZÀ-ÖØ-öø-ÿ] matches 1 character, any within those 5 ranges.

And those 5 ranges cover all the letters (alphabets) of the 2 most used Unicode blocks :

• Basic Latin ( \u0000a \u007F), and
• Latin-1 Supplementary ( \u0080a \u00FF).

A-Zand a-zin the Basic Latin block: ¹

<sup>* Characters from \u0000 to \u001F ( control codes C0 ), which are not printable, are not displayed.
* Characters from \u0080 to \u009F ( control codes C1 ) are not displayed, and are also not printable.</sup>

À-Ö, Ø-öand ø-ÿin the Latino-1 Supplementary block: ²

I mean, ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyzÀÁÂÃÄÅÆÇÈÉÊËÌÍÎÏÐÑÒÓÔÕÖØÙÚÛÜÝÞßàáâãäåæçèéêëìíîïðñòóôõöøùúûüýþÿ.

Grades:

• The middle hyphen ( -) is not included. Hyphens in the character class only build ranges. If you wanted to include it, you would add it escaped like \-inside the square brackets.
  _{- Another option is to add the hyphen, without escaping it, at the beginning or end of the class, where they never form a range.}

• Spaces and other punctuation marks are also not included.

• Does not include all Latin letters with diacritics. Some very rare characters from the Extended Latin-A ( \u0100a \u017F) or Extended Latin-B ( \u0180a \u024F) blocks are not in the class.

  

  Examples: ĩ, Ō, ũ, ŷ, ƒ, ǎ, ȩ, Ɇ, etc.

• Letters from the Additional Extended Latin Block , nor from the Greek , Cyrillic , Armenian , Hebrew , or Arabic alphabets , among many others , are obviously not included either .

• The Match Unicode Block Range page generates a character class from the Unicode blocks that are selected.

• If you only want Spanish letters, the class is [A-Za-zÁÉÍÓÚÜÑáéíóúüñ]o, which is the same, ignoring upper and lower case:

  /^[a-záéíóúüñ]+$/i
  

• While there is no other way in JavaScript (which has the worst regex implementation), other languages ​​allow more friendly ways to match letters.

In theoretical computation and theory of formal languages, a regular expression, also known as regex, regexp1​ or rational expression,2​3​ is a sequence of characters that forms a search pattern, mainly used for searching for patterns of character strings. or substitution operations.

In your example:

\       Buscará coincidencias
^       Coincide con el principio de la entrada
$       Busca el final de la entrada
+       Busca el carácter precedente 1 o más veces
[xyz]   Grupo de caracteres