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

6. Regex Pieces Operation Example Meaning Character class [Rr] or [rR]
[abcd]
[\^]
R or r
Exactly one of a, b, c, or d
Just carat (^)
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
wx|xyz
One of the strings wx or xyz
Star
[abc]*
Any number of a’s b’s and c’s at all
Plus
[abc]+
At least one of a’s, b’s, and c’s

7. Regex Pieces, Cont.
Operation
Example
Meaning

8. [Rr]? ‘’ R r
(yz | xyz)?

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

10. Give strings which match:
[Dr]oc[^0-9][3-5!]
rock!
DocǓ4
[3-5!]
A character (between 3 and 5) or !

11. Something we can’t do with [], *, +, ?
His or hers
Hi?e?r?s
His|hers
H (i|er) s

12. Give strings which match:
fl[ou]*r? fl
flour
floor
flu
flo
floo
flououou
flooooooooooooor
floouuuouuuuuuuuuo
flur
flr
Fl[ou]*r
Flr | fl[ou]r | fl[ou][ou]r …

13. Give an Expression to match
3 words separated by spaces
[a-z]* [a-z]* [a-z]*
The box
[a-z]+ [a-z]+ [a-z]+

14. Give an Expression to match
nate and brunelle within 3 words of one another
nate man myth legend brunelle
nate the rock brunelle
Nate brunelle
nate ([a-z]+ )?([a-z]+ )?([a-z]+ )?brunelle

15. In python import re Compile Operate Match Object search finditer
Similar to string.find()
finditer
Findall
0 parentheses: m.group()
1 paren: m.group(1)
2+ paren: m.groups()
Match Object
group
start
end
groups