I am trying to create regular expression that will validate simple patterns. Here a lists of valid and invalid patterns.

Valid:

foo, foo.bar, foo.bar.baz,
*, *.foo, foo.*, *.foo.*, *.foo.bar, foo.*.bar, foo.bar.*, *.foo.bar.*,
**, **.foo, foo.**, **.foo.**, **.foo.bar, foo.**.bar, foo.bar.**, **.foo.bar.**

And invalid:

<empty string>, null, false, true, foo bar baz,
.foo, foo., .foo.,
***, ***.foo, foo.***, ***.foo.*, foo.***.bar

So allowed marks are * and **. I spent a lot of time trying to write a regexp for those patterns, and best solution was:

/(\.|^)(\w+|\*{1,2})(?=\.|$)/

But it doesn't check whole patten, just finds it parts, so I cannot validate using preg_match. Another problem is that .foo pattern is valid according to that regexp, but it shouldn't because of dot in the beginning.

What did I miss?

Use this regex:

/^(\*{1,2}|[a-z]+)(\.(\*{1,2}|[a-z]+))*$/i

Here is sample code to test it by validating that all of the "good" patterns match, but not the "bad" ones. It correctly handles every one of the examples you gave.

$good_data = array(
    'foo',
    'foo.bar',
    'foo.bar.baz',
    '*',
    '*.foo',
    'foo.*',
    '*.foo.*',
    '*.foo.bar',
    'foo.*.bar',
    'foo.bar.*',
    '*.foo.bar.*',
    '**',
    '**.foo',
    'foo.**',
    '**.foo.**',
    '**.foo.bar',
    'foo.**.bar',
    'foo.bar.**',
    '**.foo.bar.**'
    );
$bad_data = array(
    '',
    null,
    false,
    true,
    'foo bar baz',
    '.foo',
    'foo.',
    '.foo.',
    '***',
    '***.foo',
    'foo.***',
    '***.foo.*',
    'foo.***.bar'
    );
$regex = "/^(\*{1,2}|[a-z]+)(\.(\*{1,2}|[a-z]+))*$/i";
foreach($good_data as $data) {
    echo (preg_match($regex, $data) ? "pass" : "FAIL: $data")."<br/>";
}
foreach($bad_data as $data) {
    echo (!preg_match($regex, $data) ? "pass" : "FAIL: $data")."<br/>";
}

more