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. All UVA computing IDs
Operation
Example
Meaning
Count Range
{3, 5}
[ab]{2,3}
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
All UVA computing IDs
2-3 letters, number, 1-3 letters
[a-z]{2,3}[1-9][a-z]{1,3}

8. 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]?

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

10. In python import re Compile Operate Match Object search
Similar to string.find() # gives only the first match
Finditer # gives a collection of matches
Findall
0 parentheses: m.group()
1 paren: m.group(1)
2+ paren: m.groups()
Match Object
group
start
end
groups