jquery - Javascript string validation. How to write a character only once in string and only in the start? -

i writing validation phone numbers. need allow users write + character in begining of input field , prevent users writing later in field.

in other words:

+11111111 - right,

111111111 - right,

+111+111+ - false,

1111+111+ - false

the problem need perform validation while typing. result cannot analyse whole string after submision, not possible fetch position of + character because 'keyup' returns 0.

i have tryed many approaches, 1 of them:

  $('#signup-form').find('input[name="phone"]').on('keyup', function(e) {         // prevent typing letters         $(this).val($(this).val().replace(/[^\d.+]/g, ''));          var textval = $(this).val();          // check if + character occurs         if(textval === '+'){           // remove + occurring twice           // check if + character not first           if(textval.indexof('+') > 0){              var newvalrem = textval.replace(/\+/, '');              $(this).val(newvalrem);           }          } }); 

when trying replace + character empty string replaced once not enough, because user might type cople of times mistake.

here link fiddle: https://jsfiddle.net/johannesmt/rghlowxq/6/

please give me hint in situation. thanks!

to current code fix (@thomas mauduit-blin right there lot more here allow plus symbol @ beginning only), may remove plus symbols preceded character. capture character , restore backreference in replacement pattern:

$(this).val($(this).val().replace(/[^\d.+]|(.)\++/g, '$1')); 

see updated fiddle , regex demo.

the pattern updated (.)\++ alternative. (.) captures character newline group 1 followed 1 or more plus symbols, , contents of group 1 placed during replacement of $1 backreference.