Adapting PHP regular expression validation phone for javascript

The fact is that the + symbol is a service symbol of regular expressions ; therefore, to use it as a regular character, it must be escaped with a backslash ( \ ):

 /^(380|\+380)\d{9}$/ 

Check it out ...

1. At first, simply using the pattern attribute ...

HTML:

 <form action="//google.com/search" target="_blank"> <input name="q" required pattern="(380|\+380)\d{9}"> <button>GO</button> </form> 

Works: https://jsfiddle.net/q8wt17y3/

2. Now, for reliability, and through JavaScript ...

HTML:

 <form action="//google.com/search" target="_blank"> <input name="q"> <button>GO</button> </form> 

Javascript:

 document.forms[0].onsubmit = function () { if ( !this.q.value.match( /^(380|\+380)\d{9}$/ ) ) { alert('No match with pattern!'); return false; } }; 

Works: https://jsfiddle.net/4cfko1gq/

By the way, in the HTML-attribute of pattern designation of the beginning ( ^ ) and end of the data ( $ ) can be omitted, since they are set by default. But in Javascript they are needed if you want to mark the beginning and / or end of the data.

Also note that if you create a regular expression using the new RegExp() constructor, then instead of one backslash ( \ ), you need to specify two ( \\ ).

The fact is that the constructor new RegExp() takes as its first parameter a regular string; and to put a backslash on a line, it needs to be screened; This is due to the fact that when the interpreter finds a backslash in a string, it thinks that it is a screened sequence (for example, the newline is \n ).