Javascript REGEX: To allow ONLY specific characters, with a bit of addition conditions?

Question

Let's say i want to catch a string, with following conditions :

• ONLY these four specific characters are allowed: h , i , , !
• Any of them can appear multiple times
• String MUST start with h
• (Most importantly) h and i MUST present together (as hi ) at least 1 time.

Therefore, accepted (expected) ones could be:

• hi
• hi!
• hhi !!
• h ihi !

Non-acceptable , for example:

• ih! <-- becos: no hi
• hi <-- becos: no hi (together)
• !hi <-- becos: didn't start with h
• hi! x hi! x <-- becos: external character(s)

Here is what i tried so far:

var pattern = /^[h]+[i]+[ !]*/i;

pattern.test("hi"); //true <--- expected (correct)
pattern.test("hiiih !"); //true <--- expected (correct)
pattern.test("ihi"); //false <--- expected (correct)
pattern.test(" hi"); //false <--- expected (correct)
pattern.test("hixx"); //true <--- not expected (wrong)

(Which means (so far) i can do all i need, but cannot do characters restrictions.)

May be my whole REGEX should be corrected into different approach. Please help.

Answer 1

This is the regex:

^(?=h)[hi !]*hi[hi !]*$

• ^(?=h) : the begin of the string must be followed by an h
• [hi !]* : only the character class [hi !] is acepted
• hi : at some point there is "hi"
• [hi !]*$ : continue with the character class [hi !] until reaching the end

See demo

Answer 2

var pattern = /^h+i+[hi !]*$/i;

The pattern must start with 1 or more 'h', followed by 1 or more 'i', then can contain 'hi !' in any quantity up to the end of the string.

Answer 3

Using a small lookahead and assertion for h at the beginning.

(?=.*hi)^h[hi !]+$    

JSFiddle