How can I convert ereg expressions to preg in PHP?

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The biggest change in the syntax is the addition of delimiters.

ereg('^hello', $str);
preg_match('/^hello/', $str);

Delimiters can be pretty much anything that is not alpha-numeric, a backslash or a whitespace character. The most used are generally ~, / and #.

You can also use matching brackets:

preg_match('[^hello]', $str);
preg_match('(^hello)', $str);
preg_match('{^hello}', $str);
// etc

If your delimiter is found in the regular expression, you have to escape it:

ereg('^/hello', $str);
preg_match('/^\/hello/', $str);

You can easily escape all delimiters and reserved characters in a string by using preg_quote:

$expr = preg_quote('/hello', "https://stackoverflow.com/");
preg_match('/^'.$expr."https://stackoverflow.com/", $str);

Also, PCRE supports modifiers for various things. One of the most used is the case-insensitive modifier i, the alternative to eregi:

eregi('^hello', 'HELLO');
preg_match('/^hello/i', 'HELLO');

You can find the complete reference to PCRE syntax in PHP in the manual, as well as a list of differences between POSIX regex and PCRE to help converting the expression.

However, in your simple example you would not use a regular expression:

stripos($str, 'hello world') === 0

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