Matching fixed-format numbers and optional string in regex

Question

Regex newbie! I would like to validate a time string in the format HH:MM with an optional space and AM or PM suffix.

Example: 10:30 or 10:30 AM will both be valid.

Here's what I have so far which is failing:

  $test = '12:34';
  if(!preg_match("/^\d{2}:\d{2}?\s(AM|PM)$/", $test))
  {
      echo "bad format!";
  }

In addition, is it possible to validate that the HH or MM values are <= 12 and <=60 respectively within the same regex?

Regards, Ben.

Gumbo · Accepted Answer · 2011-01-22 13:28:19Z

3

Try this:

/^\d{2}:\d{2}(?:\s?[AP]M)?$/

Here the last part (?:\s?[AP]M)? is an optional, non-capturing group that may start with an optional whitespace character and is then followed by either AM or PM.

For the number ranges replace the first \d{2} by (?:0\d|1[012]) and the second by (?:[0-5]\d|60).

answered Jan 22, 2011 at 13:28

Gumbo

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4 Comments

Gumbo Over a year ago

@Ben: You’re welcome! By the way, minutes would only go from 0–59 as 60 already is the next hour. And as for the hours, 12=0 in the next 12-hour period.

Ben Over a year ago

Can I trouble you for a little more info? Im trying to support 24 hour time format so the first two digits i.e. can be <=24, however I do not understand the syntax fully - (?:0\d|1[012]). Can you advise on how this breaks down? Many thanks, Ben.

Gumbo Over a year ago

@Ben: (?:0\d|1[012]) is a non-capturing group (?:…) containing an alternation of either a 0 followed by a digit, or a 1 followed by one of 0, 1, and 2. So for 24 hours period you would use (?:[01]\d|2[0-4]) (or 2[0-3] for 20 to 23).

Ben Over a year ago

Got it! Thank you, the explanation has been very helpful indeed. Regards, Ben.

codaddict · Accepted Answer · 2011-01-22 13:42:24Z

0

Alternative to using regular expression for matching valid minute and hour you can capture them and then use comparison operators as:

$test = '12:14';
if( preg_match("/^(\d{2}):(\d{2})(?:\s?AM|PM)?$/", $test,$m) &&
    $m[1] >= 0 && $m[1] < 12 &&  
    $m[2] >= 0 && $m[2] < 60) {
        echo "good format";
} else {
        echo "bad format!";
}

answered Jan 22, 2011 at 13:42

codaddict

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