Regular Expressions II

“.” is used to refer to any character. So

9.11

will match the lines

its stupid the post 9-11 rules
if any 1 of us did 9/11 we would have been caught in days.
NetBios: scanning ip 203.169.114.66
Front Door 9:11:46 AM
Sings: 0118999881999119725...3 !

This does not mean “pipe” in the context of regular expressions; instead it translates to “or”; we can use it to combine two expressions, the subexpressions being called alternatives

flood|fire

will match the lines

is firewire like usb on none macs?
the global flood makes sense within the context of the bible
yeah ive had the fire on tonight
... and the floods, hurricanes, killer heatwaves, rednecks, gun nuts, etc.


We can include any number of alternatives...

flood|earthquake|hurricane|coldfire

will match the lines

Not a whole lot of hurricanes in the Arctic.
We do have earthquakes nearly every day somewhere in our State
hurricanes swirl in the other direction
coldfire is STRAIGHT!
’cause we keep getting earthquakes

The alternatives can be real expressions and not just literals

^[Gg]ood|[Bb]ad

will match the lines

good to hear some good knews from someone here
Good afternoon fellow american infidels!
good on you-what do you drive?
Katie... guess they had bad experiences...
my middle name is trouble, Miss Bad News

Subexpressions are often contained in parentheses to constrain the alternatives

^([Gg]ood|[Bb]ad)

will match the lines

bad habbit
bad coordination today
good, becuase there is nothing worse than a man in kinky underwear
Badcop, its because people want to use drugs
Good Monday Holiday
Good riddance to Limey

The question mark indicates that the indicated expression is optional

[Gg]eorge( [Ww]\.)? [Bb]ush

will match the lines

i bet i can spell better than you and george bush combined
BBC reported that President George W. Bush claimed God told him to invade I
a bird in the hand is worth two george bushes

In the following

[Gg]eorge( [Ww]\.)? [Bb]ush

we wanted to match a “.” as a literal period; to do that, we had to “escape” the metacharacter, preceding it with a backslash In general, we have to do this for any metacharacter we want to include in our match

The * and + signs are metacharacters used to indicate repetition; * means “any number, including none, of the item” and + means “at least one of the item”

(.*)

will match the lines

anyone wanna chat? (24, m, germany)
hello, 20.m here... ( east area + drives + webcam )
(he means older men)
()

The * and + signs are metacharacters used to indicate repetition; * means “any number, including none, of the item” and + means “at least one of the item”

[0-9]+ (.*)[0-9]+

will match the lines

working as MP here 720 MP battallion, 42nd birgade
so say 2 or 3 years at colleage and 4 at uni makes us 23 when and if we fin
it went down on several occasions for like, 3 or 4 *days*
Mmmm its time 4 me 2 go 2 bed

{ and } are referred to as interval quantifiers; the let us specify the minimum and maximum number of matches of an expression

[Bb]ush( +[^ ]+ +){1,5} debate

will match the lines

Bush has historically won all major debates he’s done.
in my view, Bush doesn’t need these debates..
bush doesn’t need the debates? maybe you are right
That’s what Bush supporters are doing about the debate.
Felix, I don’t disagree that Bush was poorly prepared for the debate.
indeed, but still, Bush should have taken the debate more seriously.
Keep repeating that Bush smirked and scowled during the debate

• m,n means at least m but not more than n matches
• m means exactly m matches
• m, means at least m matches

• In most implementations of regular expressions, the parentheses not only limit the scope of alternatives divided by a “|”, but also can be used to “remember” text matched by the subexpression enclosed
• We refer to the matched text with \1, \2, etc.

So the expression

+([a-zA-Z]+) +\1 +

will match the lines

time for bed, night night twitter!
blah blah blah blah
my tattoo is so so itchy today
i was standing all all alone against the world outside...
hi anybody anybody at home
estudiando css css css css.... que desastritooooo

The * is “greedy” so it always matches the longest possible string that satisfies the regular expression. So

^s(.*)s

matches

sitting at starbucks
setting up mysql and rails
studying stuff for the exams
spaghetti with marshmallows
stop fighting with crackers
sore shoulders, stupid ergonomics

The greediness of * can be turned off with the ?, as in

^s(.*?)s$

• Regular expressions are used in many different languages; not unique to R.
• Regular expressions are composed of literals and metacharacters that represent sets or classes of characters/words
• Text processing via regular expressions is a very powerful way to extract data from “unfriendly” sources (not all data comes as a CSV file)
• Used with the functions grep,grepl,sub,gsub and others that involve searching for text strings (Thanks to Mark Hansen for some material in this lecture.)