Learn Regex in 4 tweets :)

cat matches cat
ca+t matches caaaaaaaaaaaat but not ct
ca*t matches caaaaaaaaaaaat and also ct
ca{2,4} matches caat, caaat and caaaat
c(at)+ matches catatatatatat
c(at|orn) matches cat and corn
c[ea] matches cat and cet
c[ea]+ matches caaaat and ceeet

c[A-C0-9] matches cAt, cBt, cCt, c8t etc.
c.t matches cat, c&t, c2t (any char between c and t)
c.+t matches c3%x4t (any number of any chars)
c.*t matches c3%x4t and as well as ct
^ denotes start of a string, $ denotes the end
^a+cat will match aaacat in aaacat but not in bbaaacat

cat$ will match cat in aaacat but not in aaacats
^cat$ will match only and only this string i.e. cat

\\d is for digits, \\w for alphanumeric chars, \\s is for white space chars & line breaks
\\D is for non-digits, \\W for non-alphamueric chars and \\s is for non-white space chars

\\t for tabs, \\r for carriage return and \\n for newline

Yes, c\\d+t matches c2784t
Yes, c\\s+ matches c t
Yes, c\\D+ matches cxxxt ca2t

Using .*w vs .*?w on xxsomethingnew@1234wxx
.*w returns somethingnew@1234w (longest match)
.*w? returns somethingnew (shortest match)

Sorry guys, I made 4 typos. I forgot to add "t" in the end of these 3 patterns.

> c[ea]t matches cat and cet
> c[ea]+t matches caaat, ceeet (also caeeeaaeat)
> c[A-C0-9]t matches cAt, cBt, cCt, c8t etc.
> .*w and .*w? will match "xx" in the starting of xxsomethingnew.... as well

Since it blew up, I am extending this cheatsheet :)

Grouping
() are capturing groups. (ca+t)(dog) will match caaatdog.
Groups can be accessed in Python as follows:
http://match.group (0) = caaatdog
http://match.group (1) = caaat
http://match.group (2) = dog

Groups can be referenced

(ca+t)dog\\1 matches caaaatdogcaaaat where \\1 is obviously the second (arrays start with 0) group

If you don't want to capture a group add ?: in the start e.g. (?:ca+t)dog

^ is also a MATCH-EVERYTHING-EXCEPT-THESE-CHARS operator.
[^pc]orn will match zorn, 7orn, $orn etc. but not porn or corn

? also serves as OPTIONAL-MATCH operator

cat(corn)?dog will match both catdog and catcorndog i.e. corn is optional.

Look-aheads and Look-behinds

cat(?=dog) matches cat in catdog not in catxyz
cat(?!dog) matches cat in catxyz not in catdog
(?<=cat)dog matches dog in catdog not in xyzdog
(?<!cat)dog matches dog in xyzdog not in catdog

Hopefully, I won't need to add another tweet to this thread so let's end it with this cute catto

Also, check out my Github: https://github.com/s0md3v 

Here comes the impossible, only match even number of cats.

(?<!cat)(?:catcat)*(?!cat)

PS: Non-matching strings return an empty match, you need to handle that in your program.