1. Nate Brunelle Today: Regular Expressions
CS1110
Nate Brunelle
Today: Regular Expressions

2. Questions?

3. String.find()
Takes a string as an argument, and if exactly that string appears, give its index
Mystring.find(“Purple Elephant”)
“purple elephant”.find(“Purple Elephant”)
“the elephant was purple”

4. Wildcards [Rr]ugs?[^a-zA-Z] Match on/ find: Will not match on/find:
Rugged
rugged
We might want:
A way of saying r or R å
Maybe there’s an s ç
Something that’s not a letter ê
åugçê
[Rr]ugs?[^a-zA-Z]

5. he she it they went to the store s?h?e?i?t? Alternation (or)
Sit
Alternation (or) |
s?he|it|they
(she|he|it|they) went to the store
she went to the store
he went to the store
it went to the store
they went to the store

6. Star vs plus vs ? Spo?ky Spo*ky Spo+ky Spky Spoky Spooky Spoooky
Spooooky
Spoooooooooooooky …
Spo+ky

7. R string
“\””
r“\”” -> error
r“\”this” -> error
r“\n” -> \n

8. Regex Pieces Operation Example Meaning Character class [Rr] or [rR]
[abcd]
[\^A]
R or r
Exactly one of a, b, c, or d
Just carat (^) or A
Character Range
[a-z]
[a-zA-Z]
[0-9]
Exactly one character “between” a and z
“between” a and z or “between” A and Z
Any one digit
Negative character class
[^a]
[^a-zA-Z]
[^\^]
Any one character that’s not an a
Any one character that’s not a letter any one character that’s not a carat
Optional Quantifier s?
[Rr]?
Maybe there’s an s, 0 or 1 s
Either have one of R or r or neither
OR, alternation
wx|xyz
s?he|it
One of the strings wx or xyz
Matches one of the two regexes
Star
[abc]*
Any number of a’s b’s and c’s at all
0 or more copies of…
Plus
[abc]+
At least one of a’s, b’s, and c’s
1 or more copies of…

9. Regex Pieces, Cont. All UVA computing IDs
Operation
Example
Meaning
Count Range
{3, 5}
[ab]{2,3}
[abc]{5}
Between 3 and 5 (inclusive) copies of.
aa, ab, ba, bb, aaa, aab, abb, baa, …
End of Text $
This is some text#
Beginning of Text ^
#This is some text
Word Boundary \b
#This# #is# #some# #text#
Anything .
Any one character .*
Any number of characters
All UVA computing IDs
2-3 letters, number, 1-3 letters
[a-z]{2,3}[2-9][a-z]{1,3}

10. Give an Expression to match
All UVA computing IDs
2-3 letters, number, 1-3 letters
[a-z] [a-z] [a-z]?[2-9] [a-z] [a-z]? [a-z]?

11. What does a for loop look like?
for [variable] in [collection]:
Variable: [a-zA-Z]+
[0, 1, 5, 9]

12. import re finder = re.compile Use the finder Match Object search
Similar to string.find(), gives just the first matching instance
finditer
Gives a collection of match objects
findall
I list containing:
0 parentheses: m.group()
1 paren: m.group(1)
2+ paren: m.groups()
Match Object
Group
The text we matched on
start
end
groups

13. Writing a regex
Write down some examples of strings you want to match, and some examples of similar strings that you don’t want to match
Want to match: njb2b, mst3k, aaa8bbb, aa4aa
Don’t want to match: a2b, njb2, 7bb
Going left-to-right through your examples, try to come up with the rules that will match/not match on the correct strings

14. Regex for phone numbers
((3n) ?|3n-)?
3n- 4n
Area = (\([0-9]{3}\) ? | [0-9]{3}-)?
Office = [0-9]{3}-
rest = [0-9]{4}
Want to match:
(434)
Don’t want to match:
Also handle parentheses
[2-9][0-9]{2}\-([2-9][0-9]{2}\-)?[0-9]{4}|\([2-9][0-9]{2}) ? [2-9][0-9]{2}\-[0-9]{4}