PHP正则表达式：字符串必须包含X个字符N次

Question

I'm newbie in the world of regular expression.
I want to create an expression can match X character N times in a string.
for example:

X = t

N = 2

then it should match string test or tit or tt.

Answer 1

Solution using RegEx

/^([^t]*t[^t]*){2}$/

Where N = 2

• ^ Anchors the regex at the start of the string.

• [^t]* Negated character class, matches anything other than t. * Quntifies it any number of times

• t Which is followed by a t

• [^t]* Further followed by anything other than t

• {2} Quantifies the pattern 2 times. That is it ensures that not t zero or more times followed by t and followed by not t zero or more time. The entire thing must occure 2 times

• $ Anchors the regex at the end of the string.

Regex Demo

Test

$X='t';
$N=2;

$regex="/^([^".$X."]*".$X."[^".$X."]*){".$N."}$/";

echo preg_match($regex, "test" );
// => True

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Solution using substr_count

The substr_count function returns the number of occurence of an substring in a string

$X='t';
$N=2;
if ( substr_count('test', $X ) == $N )
//The string is ok
else
//The string is not ok