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I am extracting a character in a Javascript string with:
var first = str.charAt(0);
and I would like to check whether it is a letter. Strangely, it does not seem like such functionality exists in Javascript. At least I cannot find it.
How can I test this?
I am extracting a character in a Javascript string with:
var first = str.charAt(0);
and I would like to check whether it is a letter. Strangely, it does not seem like such functionality exists in Javascript. At least I cannot find it.
How can I test this?
Share Improve this question edited Sep 15, 2014 at 16:38 Jérôme Verstrynge asked Mar 25, 2012 at 18:22 Jérôme VerstryngeJérôme Verstrynge 59.5k95 gold badges294 silver badges466 bronze badges 15 | Show 10 more comments18 Answers
Reset to default 220I don't believe there is a built-in function for that. But it's easy enough to write with a regex
function isLetter(str) {
return str.length === 1 && str.match(/[a-z]/i);
}
With respect to those special characters not being taken into account by simpler checks such as /[a-zA-Z]/.test(c), it can be beneficial to leverage ECMAScript case transformation (toUpperCase). It will take into account non-ASCII Unicode character classes of some foreign alphabets.
function isLetter(c) {
return c.toLowerCase() != c.toUpperCase();
}
NOTE: this solution will work only for most Latin, Greek, Armenian and Cyrillic scripts. It will NOT work for Chinese, Japanese, Arabic, Hebrew and most other scripts.
if( char.toUpperCase() != char.toLowerCase() )
Will return true only in case of letter
As point out in below comment, if your character is non English, High Ascii or double byte range then you need to add check for code point.
if( char.toUpperCase() != char.toLowerCase() || char.codePointAt(0) > 127 )
ES6 supports unicode-aware regular expressions.
RegExp(/^\p{L}/,'u').test(str)
This works for all alphabets.
Unfortunately, there is a bug in Firefox (will be fixed in version 78) that prevents this from being used universally. But if you can control your runtime environment and it supports it (e.g. Node.js), this is a straightforward, comprehensive solution.
Atlernatively, XRegExp provides a polyfill of modern regular expression to all browsers.
I believe this plugin has the capabilities you seek: http://xregexp.com/plugins/ (github link: https://github.com/slevithan/xregexp)
With it you can simply match all unicode letters with \p{L}.
Read the header of this source file to see which categories it supports: http://xregexp.com/plugins/xregexp-unicode-categories.js
How about using ASCII codes?
let n = str.charCodeAt(0);
let strStartsWithALetter = (n >= 65 && n < 91) || (n >= 97 && n < 123);
So we got a first character using charAt() function then try to match it using regular expression to check if there is a letter lowercased starting from a to z and return its results as a boolean
Boolean('#Foo'.charAt(0).match(/[a-zA-Z]/)) // false
Boolean('Foo'.charAt(0).match(/[a-zA-Z]/)) // true
This solution works with special characters too, for instance é, è, ê, ü, ö, à
2 steps:
- Remove the accents, based on this answer: Remove accents/diacritics in a string in JavaScript
- Check if a to z character, using regex or unicode (your choice)
Live demos of my solution:
- using global functions
- using javascript module pattern
Note: I posted the solution that uses global functions as it's probably the simplest to understand. But do look into "javascript module pattern" if you want better code (cleaner, easier to maintain and extend), see impressivewebs.com/my-current-javascript-design-pattern and also this YouTube video (presentation by Paul Irish).
var defaultDiacriticsRemovalap = [
{'base':'A', 'letters':'\u0041\u24B6\uFF21\u00C0\u00C1\u00C2\u1EA6\u1EA4\u1EAA\u1EA8\u00C3\u0100\u0102\u1EB0\u1EAE\u1EB4\u1EB2\u0226\u01E0\u00C4\u01DE\u1EA2\u00C5\u01FA\u01CD\u0200\u0202\u1EA0\u1EAC\u1EB6\u1E00\u0104\u023A\u2C6F'},
{'base':'AA','letters':'\uA732'},
{'base':'AE','letters':'\u00C6\u01FC\u01E2'},
{'base':'AO','letters':'\uA734'},
{'base':'AU','letters':'\uA736'},
{'base':'AV','letters':'\uA738\uA73A'},
{'base':'AY','letters':'\uA73C'},
{'base':'B', 'letters':'\u0042\u24B7\uFF22\u1E02\u1E04\u1E06\u0243\u0182\u0181'},
{'base':'C', 'letters':'\u0043\u24B8\uFF23\u0106\u0108\u010A\u010C\u00C7\u1E08\u0187\u023B\uA73E'},
{'base':'D', 'letters':'\u0044\u24B9\uFF24\u1E0A\u010E\u1E0C\u1E10\u1E12\u1E0E\u0110\u018B\u018A\u0189\uA779'},
{'base':'DZ','letters':'\u01F1\u01C4'},
{'base':'Dz','letters':'\u01F2\u01C5'},
{'base':'E', 'letters':'\u0045\u24BA\uFF25\u00C8\u00C9\u00CA\u1EC0\u1EBE\u1EC4\u1EC2\u1EBC\u0112\u1E14\u1E16\u0114\u0116\u00CB\u1EBA\u011A\u0204\u0206\u1EB8\u1EC6\u0228\u1E1C\u0118\u1E18\u1E1A\u0190\u018E'},
{'base':'F', 'letters':'\u0046\u24BB\uFF26\u1E1E\u0191\uA77B'},
{'base':'G', 'letters':'\u0047\u24BC\uFF27\u01F4\u011C\u1E20\u011E\u0120\u01E6\u0122\u01E4\u0193\uA7A0\uA77D\uA77E'},
{'base':'H', 'letters':'\u0048\u24BD\uFF28\u0124\u1E22\u1E26\u021E\u1E24\u1E28\u1E2A\u0126\u2C67\u2C75\uA78D'},
{'base':'I', 'letters':'\u0049\u24BE\uFF29\u00CC\u00CD\u00CE\u0128\u012A\u012C\u0130\u00CF\u1E2E\u1EC8\u01CF\u0208\u020A\u1ECA\u012E\u1E2C\u0197'},
{'base':'J', 'letters':'\u004A\u24BF\uFF2A\u0134\u0248'},
{'base':'K', 'letters':'\u004B\u24C0\uFF2B\u1E30\u01E8\u1E32\u0136\u1E34\u0198\u2C69\uA740\uA742\uA744\uA7A2'},
{'base':'L', 'letters':'\u004C\u24C1\uFF2C\u013F\u0139\u013D\u1E36\u1E38\u013B\u1E3C\u1E3A\u0141\u023D\u2C62\u2C60\uA748\uA746\uA780'},
{'base':'LJ','letters':'\u01C7'},
{'base':'Lj','letters':'\u01C8'},
{'base':'M', 'letters':'\u004D\u24C2\uFF2D\u1E3E\u1E40\u1E42\u2C6E\u019C'},
{'base':'N', 'letters':'\u004E\u24C3\uFF2E\u01F8\u0143\u00D1\u1E44\u0147\u1E46\u0145\u1E4A\u1E48\u0220\u019D\uA790\uA7A4'},
{'base':'NJ','letters':'\u01CA'},
{'base':'Nj','letters':'\u01CB'},
{'base':'O', 'letters':'\u004F\u24C4\uFF2F\u00D2\u00D3\u00D4\u1ED2\u1ED0\u1ED6\u1ED4\u00D5\u1E4C\u022C\u1E4E\u014C\u1E50\u1E52\u014E\u022E\u0230\u00D6\u022A\u1ECE\u0150\u01D1\u020C\u020E\u01A0\u1EDC\u1EDA\u1EE0\u1EDE\u1EE2\u1ECC\u1ED8\u01EA\u01EC\u00D8\u01FE\u0186\u019F\uA74A\uA74C'},
{'base':'OI','letters':'\u01A2'},
{'base':'OO','letters':'\uA74E'},
{'base':'OU','letters':'\u0222'},
{'base':'OE','letters':'\u008C\u0152'},
{'base':'oe','letters':'\u009C\u0153'},
{'base':'P', 'letters':'\u0050\u24C5\uFF30\u1E54\u1E56\u01A4\u2C63\uA750\uA752\uA754'},
{'base':'Q', 'letters':'\u0051\u24C6\uFF31\uA756\uA758\u024A'},
{'base':'R', 'letters':'\u0052\u24C7\uFF32\u0154\u1E58\u0158\u0210\u0212\u1E5A\u1E5C\u0156\u1E5E\u024C\u2C64\uA75A\uA7A6\uA782'},
{'base':'S', 'letters':'\u0053\u24C8\uFF33\u1E9E\u015A\u1E64\u015C\u1E60\u0160\u1E66\u1E62\u1E68\u0218\u015E\u2C7E\uA7A8\uA784'},
{'base':'T', 'letters':'\u0054\u24C9\uFF34\u1E6A\u0164\u1E6C\u021A\u0162\u1E70\u1E6E\u0166\u01AC\u01AE\u023E\uA786'},
{'base':'TZ','letters':'\uA728'},
{'base':'U', 'letters':'\u0055\u24CA\uFF35\u00D9\u00DA\u00DB\u0168\u1E78\u016A\u1E7A\u016C\u00DC\u01DB\u01D7\u01D5\u01D9\u1EE6\u016E\u0170\u01D3\u0214\u0216\u01AF\u1EEA\u1EE8\u1EEE\u1EEC\u1EF0\u1EE4\u1E72\u0172\u1E76\u1E74\u0244'},
{'base':'V', 'letters':'\u0056\u24CB\uFF36\u1E7C\u1E7E\u01B2\uA75E\u0245'},
{'base':'VY','letters':'\uA760'},
{'base':'W', 'letters':'\u0057\u24CC\uFF37\u1E80\u1E82\u0174\u1E86\u1E84\u1E88\u2C72'},
{'base':'X', 'letters':'\u0058\u24CD\uFF38\u1E8A\u1E8C'},
{'base':'Y', 'letters':'\u0059\u24CE\uFF39\u1EF2\u00DD\u0176\u1EF8\u0232\u1E8E\u0178\u1EF6\u1EF4\u01B3\u024E\u1EFE'},
{'base':'Z', 'letters':'\u005A\u24CF\uFF3A\u0179\u1E90\u017B\u017D\u1E92\u1E94\u01B5\u0224\u2C7F\u2C6B\uA762'},
{'base':'a', 'letters':'\u0061\u24D0\uFF41\u1E9A\u00E0\u00E1\u00E2\u1EA7\u1EA5\u1EAB\u1EA9\u00E3\u0101\u0103\u1EB1\u1EAF\u1EB5\u1EB3\u0227\u01E1\u00E4\u01DF\u1EA3\u00E5\u01FB\u01CE\u0201\u0203\u1EA1\u1EAD\u1EB7\u1E01\u0105\u2C65\u0250'},
{'base':'aa','letters':'\uA733'},
{'base':'ae','letters':'\u00E6\u01FD\u01E3'},
{'base':'ao','letters':'\uA735'},
{'base':'au','letters':'\uA737'},
{'base':'av','letters':'\uA739\uA73B'},
{'base':'ay','letters':'\uA73D'},
{'base':'b', 'letters':'\u0062\u24D1\uFF42\u1E03\u1E05\u1E07\u0180\u0183\u0253'},
{'base':'c', 'letters':'\u0063\u24D2\uFF43\u0107\u0109\u010B\u010D\u00E7\u1E09\u0188\u023C\uA73F\u2184'},
{'base':'d', 'letters':'\u0064\u24D3\uFF44\u1E0B\u010F\u1E0D\u1E11\u1E13\u1E0F\u0111\u018C\u0256\u0257\uA77A'},
{'base':'dz','letters':'\u01F3\u01C6'},
{'base':'e', 'letters':'\u0065\u24D4\uFF45\u00E8\u00E9\u00EA\u1EC1\u1EBF\u1EC5\u1EC3\u1EBD\u0113\u1E15\u1E17\u0115\u0117\u00EB\u1EBB\u011B\u0205\u0207\u1EB9\u1EC7\u0229\u1E1D\u0119\u1E19\u1E1B\u0247\u025B\u01DD'},
{'base':'f', 'letters':'\u0066\u24D5\uFF46\u1E1F\u0192\uA77C'},
{'base':'g', 'letters':'\u0067\u24D6\uFF47\u01F5\u011D\u1E21\u011F\u0121\u01E7\u0123\u01E5\u0260\uA7A1\u1D79\uA77F'},
{'base':'h', 'letters':'\u0068\u24D7\uFF48\u0125\u1E23\u1E27\u021F\u1E25\u1E29\u1E2B\u1E96\u0127\u2C68\u2C76\u0265'},
{'base':'hv','letters':'\u0195'},
{'base':'i', 'letters':'\u0069\u24D8\uFF49\u00EC\u00ED\u00EE\u0129\u012B\u012D\u00EF\u1E2F\u1EC9\u01D0\u0209\u020B\u1ECB\u012F\u1E2D\u0268\u0131'},
{'base':'j', 'letters':'\u006A\u24D9\uFF4A\u0135\u01F0\u0249'},
{'base':'k', 'letters':'\u006B\u24DA\uFF4B\u1E31\u01E9\u1E33\u0137\u1E35\u0199\u2C6A\uA741\uA743\uA745\uA7A3'},
{'base':'l', 'letters':'\u006C\u24DB\uFF4C\u0140\u013A\u013E\u1E37\u1E39\u013C\u1E3D\u1E3B\u017F\u0142\u019A\u026B\u2C61\uA749\uA781\uA747'},
{'base':'lj','letters':'\u01C9'},
{'base':'m', 'letters':'\u006D\u24DC\uFF4D\u1E3F\u1E41\u1E43\u0271\u026F'},
{'base':'n', 'letters':'\u006E\u24DD\uFF4E\u01F9\u0144\u00F1\u1E45\u0148\u1E47\u0146\u1E4B\u1E49\u019E\u0272\u0149\uA791\uA7A5'},
{'base':'nj','letters':'\u01CC'},
{'base':'o', 'letters':'\u006F\u24DE\uFF4F\u00F2\u00F3\u00F4\u1ED3\u1ED1\u1ED7\u1ED5\u00F5\u1E4D\u022D\u1E4F\u014D\u1E51\u1E53\u014F\u022F\u0231\u00F6\u022B\u1ECF\u0151\u01D2\u020D\u020F\u01A1\u1EDD\u1EDB\u1EE1\u1EDF\u1EE3\u1ECD\u1ED9\u01EB\u01ED\u00F8\u01FF\u0254\uA74B\uA74D\u0275'},
{'base':'oi','letters':'\u01A3'},
{'base':'ou','letters':'\u0223'},
{'base':'oo','letters':'\uA74F'},
{'base':'p','letters':'\u0070\u24DF\uFF50\u1E55\u1E57\u01A5\u1D7D\uA751\uA753\uA755'},
{'base':'q','letters':'\u0071\u24E0\uFF51\u024B\uA757\uA759'},
{'base':'r','letters':'\u0072\u24E1\uFF52\u0155\u1E59\u0159\u0211\u0213\u1E5B\u1E5D\u0157\u1E5F\u024D\u027D\uA75B\uA7A7\uA783'},
{'base':'s','letters':'\u0073\u24E2\uFF53\u00DF\u015B\u1E65\u015D\u1E61\u0161\u1E67\u1E63\u1E69\u0219\u015F\u023F\uA7A9\uA785\u1E9B'},
{'base':'t','letters':'\u0074\u24E3\uFF54\u1E6B\u1E97\u0165\u1E6D\u021B\u0163\u1E71\u1E6F\u0167\u01AD\u0288\u2C66\uA787'},
{'base':'tz','letters':'\uA729'},
{'base':'u','letters': '\u0075\u24E4\uFF55\u00F9\u00FA\u00FB\u0169\u1E79\u016B\u1E7B\u016D\u00FC\u01DC\u01D8\u01D6\u01DA\u1EE7\u016F\u0171\u01D4\u0215\u0217\u01B0\u1EEB\u1EE9\u1EEF\u1EED\u1EF1\u1EE5\u1E73\u0173\u1E77\u1E75\u0289'},
{'base':'v','letters':'\u0076\u24E5\uFF56\u1E7D\u1E7F\u028B\uA75F\u028C'},
{'base':'vy','letters':'\uA761'},
{'base':'w','letters':'\u0077\u24E6\uFF57\u1E81\u1E83\u0175\u1E87\u1E85\u1E98\u1E89\u2C73'},
{'base':'x','letters':'\u0078\u24E7\uFF58\u1E8B\u1E8D'},
{'base':'y','letters':'\u0079\u24E8\uFF59\u1EF3\u00FD\u0177\u1EF9\u0233\u1E8F\u00FF\u1EF7\u1E99\u1EF5\u01B4\u024F\u1EFF'},
{'base':'z','letters':'\u007A\u24E9\uFF5A\u017A\u1E91\u017C\u017E\u1E93\u1E95\u01B6\u0225\u0240\u2C6C\uA763'}
];
var diacriticsMap = {};
for (var i=0; i < defaultDiacriticsRemovalap.length; i++){
var letters = defaultDiacriticsRemovalap[i].letters.split("");
for (var j=0; j < letters.length ; j++){
diacriticsMap[letters[j]] = defaultDiacriticsRemovalap[i].base;
}
}
function removeDiacriticFromChar (char) {
return diacriticsMap[char] || char;
}
/*
* [1] Remove the accent, based on answer of https://stackoverflow.com/questions/990904/javascript-remove-accents-in-strings
* [2] Check if a to z character, using regex or unicode (your choice, here using regex)
*
*/
function isLetter(char) {
var charWithoutAccent = removeDiacriticFromChar(char); /* [1] */
return charWithoutAccent.match(/[a-z]/i); /* [2] */
}
console.log( "is 'u' is a letter? " + (isLetter('u') ? 'true' : 'false') );
console.log( "is 'ü' is a letter? " + (isLetter('ü') ? 'true' : 'false') );
console.log( "is 'à' is a letter? " + (isLetter('à') ? 'true' : 'false') );
console.log( "is 'ö' is a letter? " + (isLetter('ö') ? 'true' : 'false') );
console.log( "is 'ù' is a letter? " + (isLetter('ù') ? 'true' : 'false') );
console.log( "is 'é' is a letter? " + (isLetter('é') ? 'true' : 'false') );
console.log( "is 'é' is a letter? " + (isLetter('é') ? 'true' : 'false') );
console.log( "is 'ê' is a letter? " + (isLetter('ê') ? 'true' : 'false') );Here is a solution that works for all Unicode letters and works in all versions of JavaScript:
function isLetter(c) {
if (c.length != 1 || c == '_' || c == '$')
return false;
if (c.toUpperCase() != c.toLowerCase())
return true; // Speed up accepting latin letters
if (c.charCodeAt(0) < 128)
return false; // Speed up rejecting non-letter ASCII characters
try {
eval("function " + c + "(){}");
return true;
} catch {
return false;
}
}
The above code uses the fact that the first character of a JavaScript identifier must be a Unicode letter or _ or $, and cannot be anything else.
Here are some unit tests for the above function:
assertTrue(isLetter('A'));
assertTrue(isLetter('a'));
assertTrue(isLetter('é'));
assertFalse(isLetter('2'));
assertFalse(isLetter('_'));
assertTrue(isLetter('わ'));
You can use a combination of regex and test() to check if the first character is a letter
/^[a-z]/i.test(str.charAt(0))
You can use unicode character class escape:
/\p{L}/u.test('A'); // true
/\p{L}/u.test('京'); // true
let str = '0.23';
/\p{L}/u.test(char); // false
It matches any character that has a unicode property "Letter" (L)
It's only supported in unicode-aware mode, though:
console.log(/\p{L}/u.unicode); // should print "true" to work.
By default, most of the browsers have this mode enabled.
More about unicode character class escape and .unicode property
I`m posting here because I didn't want to post a new question. Assuming there aren't any single character declarations in the code, you can eval() the character to cause an error and check the type of the character. Something like:
function testForLetter(character) {
try {
//Variable declarations can't start with digits or operators
//If no error is thrown check for dollar or underscore. Those are the only nonletter characters that are allowed as identifiers
eval("let " + character + ";");
let regExSpecial = /[^\$_]/;
return regExSpecial.test(character);
} catch (error) {
return false;
}
}
console.log(testForLetter("!")); //returns false;
console.log(testForLetter("5")); //returns false;
console.log(testForLetter("ن")); //returns true;
console.log(testForLetter("_")); //returns false;We can check also in simple way as:
function isLetter(char){
return ( (char >= 'A' && char <= 'Z') ||
(char >= 'a' && char <= 'z') );
}
console.log(isLetter("a"));
console.log(isLetter(3));
console.log(isLetter("H"));Another possibility is using the library unicode-properties. The library supports Unicode version 12. In this example the library returns the character category for a given string. And then checks if the first character of the category equals to 'L' (which obviously stands for letter).
var unicode = require("unicode-properties");
let str = "E";
console.log(unicode.getCategory(str.charCodeAt(0)).charAt(0) === "L"); // True// to check if the given string contain alphabets
function isPangram(sentence){
let lowerCased = sentence.toLowerCase();
let letters = "abcdefghijklmnopqrstuvwxyz";
// traditional for loop can also be used
for (let char of letters){
if (!lowerCased.includes(char)) return false;
}
return true;
}
I made a function to do this:
var isLetter = function (character) {
if( (character.charCodeAt() >= 65 && character.charCodeAt() <= 90) || (character.charCodeAt() >= 97 && character.charCodeAt() <= 122) ) {
return true;
}
else{
return false;
}
}
This basically verifies in the ASCII table if the code of the character refers to a Letter.
You can see the ASCII table on this link and compare columns DEC (where is the code) and symbol: https://www.ascii-code.com/
Note: My function is true only to "simple" letters (things like "Á", "é", "ç", "Ü" it will return false... but if you needed you can adapt this function to de other conditions).
It's possible to know if the character is a letter or not by using a standard builtin function isNaN or Number.isNaN() from ES6:
isNaN('s') // true
isNaN('-') // true
isNaN('32') // false, '32' is converted to the number 32 which is not NaN
It retruns true if the given value is not a number, otherwise false.
You can simply use built in function isNaN , like this:
var char = 'a'; // can try '123', 'abc'
var checkNo =isNaN(char);
console.log(checkNo); // true本文标签： stringHow to check if character is a letter in JavascriptStack Overflow
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NaN? Or justisNan(first)I guess. – Marc Commented Mar 25, 2012 at 18:24