How to Detect Non "GSM 7 bit alphabet" characters in input field

You can put all valid characters in a string and then search the string repeatedly.

gsm = "@£$¥èéùìòÇØøÅåΔ_ΦΓΛΩΠΨΣΘΞ^{}\[~]|€ÆæßÉ!\"#¤%&'()*+,-./0123456789:;<=>?¡ABCDEFGHIJKLMNOPQRSTUVWXYZÄÖÑܧ¿abcdefghijklmnopqrstuvwxyzäöñüà";
var letter = 'a';
var letterInAlfabet = gsm.indexOf(letter) !== -1;

Make sure you get your encodings right if you use this, i.e. save your Javascript file as UTF8 and specify that it is UTF8 to the browser.

The accepted answers will work, but they suffer from complexity (using a regex) and performance (needing to search through two arrays). Here's a solution that will perform better, due to the use of a lookup Set, and a loop which will short-circuit if a non-GSM7 character is found. Unicode points are used so that different character encodings are not a problem when cutting and pasting this code.

const gsmCodePoints = new Set([
  0x000a, 0x000c, 0x000d, 
  0x0020, 0x0021, 0x0022, 0x0023, 0x0024, 0x0025, 0x0026, 0x0027, 0x0028, 0x0029, 0x002a, 0x002b, 0x002c, 0x002d, 0x002e, 0x002f,
  0x0030, 0x0031, 0x0032, 0x0033, 0x0034, 0x0035, 0x0036, 0x0037, 0x0038, 0x0039, 0x003a, 0x003b, 0x003c, 0x003d, 0x003e, 0x003f,
  0x0040, 0x0041, 0x0042, 0x0043, 0x0044, 0x0045, 0x0046, 0x0047, 0x0048, 0x0049, 0x004a, 0x004b, 0x004c, 0x004d,
  0x004e, 0x004f,
  0x0050, 0x0051, 0x0052, 0x0053, 0x0054, 0x0055, 0x0056, 0x0057, 0x0058, 0x0059,  0x005a, 0x005b, 0x005c, 0x005d, 0x005e, 0x005f, 
  0x0061, 0x0062, 0x0063, 0x0064, 0x0065, 0x0066, 0x0067, 0x0068, 0x0069, 0x006a, 0x006b, 0x006c, 0x006d, 0x006e, 0x006f, 
  0x0070, 0x0071, 0x0072, 0x0073, 0x0074, 0x0075, 0x0076, 0x0077, 0x0078, 0x0079, 0x007a, 0x007b, 0x007c, 0x007d, 0x007e,
  0x00a1, 0x00a3, 0x00a4, 0x00a5, 0x00a7,
  0x00bf,
  0x00c4, 0x00c5, 0x00c6, 0x00c7, 0x00c9,
  0x00d1, 0x00d6, 0x00d8, 0x00dc, 0x00df,
  0x00e0, 0x00e4, 0x00e5, 0x00e6, 0x00e8, 0x00e9, 0x00ec,
  0x00f1, 0x00f2, 0x00f6, 0x00f8, 0x00f9, 0x00fc,
  0x0393, 0x0394, 0x0398, 0x039b, 0x039e, 0x03a0, 0x03a3, 0x03a6, 0x03a8, 0x03a9,
  0x20ac,
]);

function isGsmMessage(message) {
  for (const s of message) {
    const codePoint = s.codePointAt(0);
    if (codePoint && !gsmCodePoints.has(codePoint)) {
      return false;
    }
  }
  return true;
}

isGsmMessage('foo'); // -> true
isGsmMessage('⚡️ bar 🔥'); // -> false

// All GSM characters
isGsmMessage('@£$¥èéùìòÇ\nØø\rÅåΔ_ΦΓΛΩΠΨΣΘΞÆæßÉ\x20!"#¤%&\'()*+,-./0123456789:;<=>?¡ABCDEFGHIJKLMNOPQRSTUVWXYZÄÖÑܧ¿abcdefghijklmnopqrstuvwxyzäöñüà\f^{}\\[~]|€'); // -> true

function isGSMAlphabet(text) {
    var regexp = new RegExp("^[A-Za-z0-9 \\r\\n@£$¥èéùìòÇØøÅå\u0394_\u03A6\u0393\u039B\u03A9\u03A0\u03A8\u03A3\u0398\u039EÆæßÉ!\"#$%&'()*+,\\-./:;<=>?¡ÄÖÑܧ¿äöñüà^{}\\\\\\[~\\]|\u20AC]*$");

    return regexp.test(text);
}

This regular expression should solve your problem.