Learn to Program 16
In this video we’ll continue learning about regular expressions. We’ll cover +, *, Greedy Matching, Lazy Matching, \b, String Boundaries ^ and $, (?m), Subexpressions and we’ll solve a bunch of problems.
If you missed the last regular expression video it is here. All of the code and a transcript of the video follows the video below.
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Code & Transcript
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import re # Did you find a match # if re.search("REGEX", yourString) # Get list of matches # print("Matches :", len(re.findall("REGEX", yourString))) # Get a pattern object # regex = re.compile("REGEX") # Substitute the match # yourString = regex.sub("substitution", yourString) # [ ] : Match what is in the brackets # [^ ] : Match anything not in the brackets # . : Match any 1 character or space # + : Match 1 or more of what proceeds # \n : Newline # \d : Any 1 number # \D : Anything but a number # \w : Same as [a-zA-Z0-9_] # \W : Same as [^a-zA-Z0-9_] # \s : Same as [\f\n\r\t\v] # \S : Same as [^\f\n\r\t\v] # {5} : Match 5 of what proceeds the curly brackets # {5,7} : Match values that are between 5 and 7 in length # ---------- Matching Zero or One ---------- randStr = "cat cats" regex = re.compile("[cat]+s?") matches = re.findall(regex, randStr) # Match cat or cats print("Matches :", len(matches)) for i in matches: print(i) # ---------- Matching Zero or More ---------- # * matches zero or more of what proceeds it randStr = "doctor doctors doctor's" # Match doctor doctors or doctor's regex = re.compile("[doctor]+['s]*") matches = re.findall(regex, randStr) print("Matches :", len(matches)) # You can do the same by setting an interval match regex = re.compile("[doctor]+['s]{0,2}") matches = re.findall(regex, randStr) print("Matches :", len(matches)) for i in matches: print(i) # ---------- PROBLEM ---------- # On Windows newlines are some times \n and other times \r\n # Create a regex that will grab each of the lines in this # string, print out the number of matches and each line longStr = '''Just some words and some more\r and more ''' print("Matches :", len(re.findall(r"[\w\s]+[\r]?\n", longStr))) matches = re.findall("[\w\s]+[\r]?\n", longStr) for i in matches: print(i) # ---------- Greedy & Lazy Matching ---------- randStr = "<name>Life On Mars</name><name>Freaks and Geeks</name>" # Let's try to grab everything between <name> tags # Because * is greedy (It grabs the biggest match possible) # we can't get what we want, which is each individual tag # match regex = re.compile(r"<name>.*</name>") matches = re.findall(regex, randStr) print("Matches :", len(matches)) for i in matches: print(i) # We want to grab the smallest match we use *?, +?, or # {n,}? instead regex = re.compile(r"<name>.*?</name>") matches = re.findall(regex, randStr) print("Matches :", len(matches)) for i in matches: print(i) # ---------- Word Boundaries ---------- # We use word boundaries to define where our matches start # and end # \b matches the start or end of a word # If we want ape it will match ape and the beginning of apex randStr = "ape at the apex" regex = re.compile(r"ape") # If we use the word boundary regex = re.compile(r"\bape\b") matches = re.findall(regex, randStr) print("Matches :", len(matches)) for i in matches: print(i) # ---------- String Boundaries ---------- # ^ : Matches the beginning of a string if outside of # a [ ] # $ : Matches the end of a string # Grab everything from the start of the string to @ randStr = "Match everything up to @" regex = re.compile(r"^.*[^@]") matches = re.findall(regex, randStr) print("Matches :", len(matches)) for i in matches: print(i) # Grab everything from @ to the end of the line randStr = "@ Get this string" regex = re.compile(r"[^@\s].*$") matches = re.findall(regex, randStr) print("Matches :", len(matches)) for i in matches: print(i) # Grab the 1st word of each line using the the multiline # code which allows for the targeting of each line after # a line break with ^ randStr = '''Ape is big Turtle is slow Cheetah is fast''' regex = re.compile(r"(?m)^.*?\s") matches = re.findall(regex, randStr) print("Matches :", len(matches)) for i in matches: print(i) # ---------- Subexpressions ---------- # Subexpressions are parts of a larger expression # If you want to match for a large block, but # only want to return part of it. To do that # surround what you want with ( ) # Get just the number minus the area code randStr = "My number is 412-555-1212" regex = re.compile(r"412-(.*)") matches = re.findall(regex, randStr) print("Matches :", len(matches)) for i in matches: print(i) # ---------- Problem ---------- # Get just the numbers minus the area codes from # this string randStr = "412-555-1212 412-555-1213 412-555-1214" regex = re.compile(r"412-(.{8})") matches = re.findall(regex, randStr) print("Matches :", len(matches)) for i in matches: print(i) # ---------- Multiple Subexpressions ---------- # You can have multiple subexpressions as well # Get both numbers that follow 412 separately randStr = "My number is 412-555-1212" regex = re.compile(r"412-(.*)-(.*)") matches = re.findall(regex, randStr) print("Matches :", len(matches)) print(matches[0][0]) print(matches[0][1]) |
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