Regular Expressions

• Regular expressions can be thought of as a combination of literals and metacharacters
• To draw an analogy with natural language, think of literal text forming the words of this language, and the metacharacters defining its grammar
• Regular expressions have a rich set of metacharacters

Simplest pattern consists only of literals. The literal “nuclear” would match to the following lines:

Ooh. I just learned that to keep myself alive after a
nuclear blast! All I have to do is milk some rats
then drink the milk. Aweosme. :}

Laozi says nuclear weapons are mas macho

Chaos in a country that has nuclear weapons -- not good.

my nephew is trying to teach me nuclear physics, or
possibly just trying to show me how smart he is
so I’ll be proud of him [which I am].

lol if you ever say "nuclear" people immediately think
DEATH by radiation LOL

The literal “Obama” would match to the following lines

Politics r dum. Not 2 long ago Clinton was sayin Obama
was crap n now she sez vote 4 him n unite? WTF?
Screw em both + Mcain. Go Ron Paul!

Clinton conceeds to Obama but will her followers listen??

Are we sure Chelsea didn’t vote for Obama?

thinking ... Michelle Obama is terrific!

jetlag..no sleep...early mornig to starbux..Ms. Obama
was moving

• Simplest pattern consists only of literals; a match occurs if the sequence of literals occurs anywhere in the text being tested

• What if we only want the word “Obama”? or sentences that end in the word “Clinton”, or “clinton” or “clinto”?

We need a way to express

• whitespace word boundaries
• sets of literals
• the beginning and end of a line
• alternatives (“war” or “peace”) Metacharacters to the rescue!

Some metacharacters represent the start of a line

^i think

will match the lines

i think we all rule for participating
i think i have been outed
i think this will be quite fun actually
i think i need to go to work
i think i first saw zombo in 1999.

$ represents the end of a line

morning$

will match the lines

well they had something this morning
then had to catch a tram home in the morning
dog obedience school in the morning
and yes happy birthday i forgot to say it earlier this morning
I walked in the rain this morning
good morning

We can list a set of characters we will accept at a given point in the match

[Bb][Uu][Ss][Hh]

will match the lines

The democrats are playing, "Name the worst thing about Bush!"
I smelled the desert creosote bush, brownies, BBQ chicken
BBQ and bushwalking at Molonglo Gorge
Bush TOLD you that North Korea is part of the Axis of Evil
I’m listening to Bush - Hurricane (Album Version)

^[Ii] am

will match

i am so angry at my boyfriend i can’t even bear to
look at him

i am boycotting the apple store

I am twittering from iPhone

I am a very vengeful person when you ruin my sweetheart.

I am so over this. I need food. Mmmm bacon...

Similarly, you can specify a range of letters [a-z] or [a-zA-Z]; notice that the order doesn’t matter

^[0-9][a-zA-Z]

will match the lines

7th inning stretch
2nd half soon to begin. OSU did just win something
3am - cant sleep - too hot still.. :(
5ft 7 sent from heaven
1st sign of starvagtion

When used at the beginning of a character class, the “^” is also a metacharacter and indicates matching characters NOT in the indicated class

[^?.]$

will match the lines

i like basketballs
6 and 9
dont worry... we all die anyway!
Not in Baghdad
helicopter under water? hmmm