Question

I would like to have a regex pattern to match smileys “:)” ,”:(” .Also it should capture repeated smileys like “:) :)” , “:) :(” but filter out invalid syntax like “:( (” .

I have this with me, but it matches “:( (”

bool( re.match("(:\()",str) ) 

I maybe missing something obvious here, and I’d like some help for this seemingly simple task.

Answer 1

I think it finally “clicked” exactly what you’re asking about here. Take a look at the below:

import re

smiley_pattern = '^(:\(|:\))+$' # matches only the smileys ":)" and ":("

def test_match(s):
    print 'Value: %s; Result: %s' % (
        s,
        'Matches!' if re.match(smiley_pattern, s) else 'Doesn\'t match.'
    )

should_match = [
    ':)',   # Single smile
    ':(',   # Single frown
    ':):)', # Two smiles
    ':(:(', # Two frowns
    ':):(', # Mix of a smile and a frown
]
should_not_match = [
    '',         # Empty string
    ':(foo',    # Extraneous characters appended
    'foo:(',    # Extraneous characters prepended
    ':( :(',    # Space between frowns
    ':( (',     # Extraneous characters and space appended
    ':(('       # Extraneous duplicate of final character appended
]

print('The following should all match:')
for x in should_match: test_match(x);

print('')   # Newline for output clarity

print('The following should all not match:')
for x in should_not_match: test_match(x);

The problem with your original code is that your regex is wrong: (:\(). Let’s break it down.

The outside parentheses are a “grouping”. They’re what you’d reference if you were going to do a string replacement, and are used to apply regex operators on groups of characters at once. So, you’re really saying:

• ( begin a group
  • :\( … do regex stuff …
• ‘)’ end the group

The : isn’t a regex reserved character, so it’s just a colon. The \ is, and it means “the following character is literal, not a regex operator”. This is called an “escape sequence”. Fully parsed into English, your regex says

• ( begin a group
  • : a colon character
  • \( a left parenthesis character
• ) end the group

The regex I used is slightly more complex, but not bad. Let’s break it down: ^(:\(|:\))+$.

^ and $ mean “the beginning of the line” and “the end of the line” respectively. Now we have …

• ^ beginning of line
  • (:\(|:\))+ … do regex stuff …
• $ end of line

… so it only matches things that comprise the entire line, not simply occur in the middle of the string.

We know that ( and ) denote a grouping. + means “one of more of these”. Now we have:

• ^ beginning of line
• ( start a group
  • :\(|:\) … do regex stuff …
• ) end the group
• + match one or more of this
• $ end of line

Finally, there’s the | (pipe) operator. It means “or”. So, applying what we know from above about escaping characters, we’re ready to complete the translation:

• ^ beginning of line
• ( start a group
  • : a colon character
  • \( a left parenthesis character
• | or
  • : a colon character
  • \) a right parenthesis character
• ) end the group
• + match one or more of this
• $ end of line

I hope this helps. If not, let me know and I’ll be happy to edit my answer with a reply.