# re --- Regular expression operations — Checking for a pair

> In this example, we'll use the following helper function to display match objects a little more gracefully def displaymatch(match): if match is None: return None return '' % (match.group(), match.groups()) Suppose you are writing a poker program where a player's hand is represented as a 5-character

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## Knowledge

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In this example, we'll use the following helper function to display match objects a little more gracefully

def displaymatch(match): if match is None: return None return '' % (match.group(), match.groups())

Suppose you are writing a poker program where a player's hand is represented as a 5-character string with each character representing a card, "a" for ace, "k" for king, "q" for queen, "j" for jack, "t" for 10, and "2" through "9" representing the card with that value.

To see if a given string is a valid hand, one could do the following

&gt;&gt;&gt; valid_hand_re = re.compile(r"^[a2-9tjqk]{5}$") &gt;&gt;&gt; displaymatch(valid_hand_re.search("akt5q")) # Valid. "" &gt;&gt;&gt; displaymatch(valid_hand_re.search("akt5e")) # Invalid. &gt;&gt;&gt; displaymatch(valid_hand_re.search("akt")) # Invalid. &gt;&gt;&gt; displaymatch(valid_hand_re.search("727ak")) # Valid. ""

That last hand, "727ak", contained a pair, or two of the same valued cards. To match this with a regular expression, one could use backreferences as such

&gt;&gt;&gt; pair_re = re.compile(r".(.).\1") &gt;&gt;&gt; displaymatch(pair_re.prefixmatch("717ak")) # Pair of 7s. "" &gt;&gt;&gt; displaymatch(pair_re.prefixmatch("718ak")) # No pairs. &gt;&gt;&gt; displaymatch(pair_re.prefixmatch("354aa")) # Pair of aces. ""

To find out what card the pair consists of, one could use the ~Match.group method of the match object in the following manner

&gt;&gt;&gt; pair_re = re.compile(r".(.).\1") &gt;&gt;&gt; pair_re.prefixmatch("717ak").group(1) '7'

# Error because prefixmatch() returns None, which doesn't have a group() method: &gt;&gt;&gt; pair_re.prefixmatch("718ak").group(1) Traceback (most recent call last): File "", line 1, in pair_re.prefixmatch("718ak").group(1) AttributeError: 'NoneType' object has no attribute 'group'

&gt;&gt;&gt; pair_re.prefixmatch("354aa").group(1) 'a'

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