Lecture 28

Regular expressions

MCS 260 Fall 2020
Emily Dumas

Reminders

  • If you haven't started Project 3, you are behind!
  • Worksheet 10 available
  • Quiz 10 coming Thursday, due Nov 2
  • Nov 3: All UIC courses canceled (Election day)
  • Nov 5: Extra TA office hours instead of discussions
  • Loose end: Recursion pros and cons

    Often can solve a problem with recursion or with loops (an iterative solution). Why use recursion?

    Pros:

    • Short code
    • Clear code

    Unclear:

    • Speed

    Cons:

    • Uses more memory

    Regular expressions

    Today we'll learn about the module re in Python, which supports a text searching language known as regular expressions or regexes.

    Some of its key functions include:

    • Searching for text matching a pattern
    • Replacing text matching a pattern

    Minimal example

    Regexes are a mini programming language for specifying patterns of text. Dialects of regex are supported in many programming languages. We'll cover the Python dialect.

    Simplest usage: Find and replace a substring.

    
    import re
    s = "Avocado is usually considered a vegetable."
    print(re.sub("vegetable","fruit",s))
        
    re.sub(pattern, replacement, string)

    The first argument of re.sub is a pattern.

    Unless it contains characters with special meaning in a regex pattern, the pattern just matches substrings equal to the pattern.

    • "vegetable" matches the string "vegetable"
    • "foo" matches the string "foo"

    raw strings

    Recall that backslash \ in a string starts an escape sequence in Python, and \\ represents a single backslash character in the string. If your string contains a lot of backslashes, you may want to disable escape sequences.

    You can do so by putting the letter r immediately before the quotation mark(s). This is known as a raw string. In a raw string, a single \ represents the \ character.

    special characters in patterns

    • . — matches any character except newline
    • \s — matches any whitespace character
    • \d — matches a decimal digit
    • + — previous item must repeat 1 or more times
    • * — previous item must repeat 0 or more times
    • ? — previous item must repeat 0 or 1 times
    • {n} — previous item must appear n times

    Example problem

    Replace any price in whole dollars (written like $2 or $1999) with the string -PRICE-.

    Note: $ is a special character. To match a dollar sign, use \$.

    Matching and searching

    What if you don't want to replace a regex, just find it?

    • re.match(pattern,string) — does string begin with a match to pattern? Return a match object or None.
    • re.search(pattern,string) — does string contain a match to the pattern? Return a match object or None.
    • re.findall(pattern,string) — return a list of all non-overlapping matches as strings.

    Match objects

    If a match is found, then the match object has a method .group() that returns the full text of the match.

    .start() and .end() return the indices where the match begins and ends in the string.

    Parentheses

    A part of a pattern in parentheses is a group. A group is treated as a unit for operators like +,*,?.

    e.g. pattern (ha)+ matches ha or haha or hahaha but does not match Haha or h or hah.

    Matched groups are available from the match object using .group(1), .group(2), etc..

    Example problem

    Find all of the phone numbers in a string that are written in the format 319-555-1012, and split each one into area code (e.g. 319), exchange (e.g. 555), and line number (e.g. 1012).

    References

    Revision history

    • 2020-10-29 Move unused slides to Lecture 29
    • 2020-10-27 Initial publication