Learn to Program 17
In this part of my tutorial I continue covering Regular Expressions. We’ll look at Back References, Back Reference Substitutions, Look Ahead, Look Behind, and Negative Look Ahead & Behind. These are thought to be confusing and I hope to clear them up.
If you missed previous parts of my Regular Expression tutorial they are here Part 1 and Part 2. The code and transcript are found 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 # ( ) : Return surrounded submatch # . : Match any 1 character or space # + : Match 1 or more of what proceeds # ? : Match 0 or 1 # * : Match 0 or More # *? : Lazy match the smallest match # \b : Word boundary # ^ : Beginning of String # $ : End of String # \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 # ($m) : Allow ^ on multiline string # ---------- Back References ---------- # A back reference allows you to to reuse the expression # that proceeds it # Grab a double word randStr = "The cat cat fell out the window" # Match a word boundary, 1 or more characters followed # by a space if it is then followed by the same # match that is surrounded by the parentheses regex = re.compile(r"(\b\w+)\s+\1") matches = re.findall(regex, randStr) print("Matches :", len(matches)) for i in matches: print(i) # ---------- Back Reference Substitutions ---------- # Replace the bold tags in the link with no tags randStr = "<a href='#'><b>The Link</b></a>" # Regex matches bold tags and grabs the text between # them to be used by the back reference regex = re.compile(r"<b>(.*?)</b>") # Replace the tags with just the text between them randStr = re.sub(regex, r"\1", randStr) print(randStr) # ---------- Another Back Reference Substitution ---------- # Receive this string randStr = "412-555-1212" # Match the phone number using multiple subexpressions regex = re.compile(r"([\d]{3})-([\d]{3}-[\d]{4})") # Output (412)555-1212 randStr = re.sub(regex, r"(\1)\2", randStr) print(randStr) # ---------- PROBLEM ---------- # Receive a string like this randStr = "https://www.youtube.com http://www.google.com" # Grab the URL and then provide the following output # using a back reference substitution # <a href='https://www.youtube.com'>www.youtube.com</a> # <a href='https://www.google.com'>www.google.com</a> regex = re.compile(r"(https?://([\w.]+))") randStr = re.sub(regex, r"<a href='\1'>\2</a>\n", randStr) print(randStr) # ---------- Look Ahead ---------- # A look ahead defines a pattern to match but not return # You define the expression to look for but not return # like this (?=expression) randStr = "One two three four" # Grab all letters and numbers of 1 or more separated # by a word boundary but don't include it regex = re.compile(r"\w+(?=\b)") matches = re.findall(regex, randStr) for i in matches: print(i) # ---------- Look Behind ---------- # The look behind looks for what is before the text # to return, but doesn't return it # It is defined like (?<=expression) randStr = "1. Bread 2. Apples 3. Lettuce" # Find the number, period and space, but only return # the 1 or more letters or numbers that follow regex = re.compile(r"(?<=\d.\s)\w+") matches = re.findall(regex, randStr) for i in matches: print(i) # ---------- Look Ahead & Behind ---------- randStr = "<h1>I'm Important</h1> <h1>So am I</h1>" # Use the look behind, get 1 or more of anything, # and use the look ahead regex = re.compile(r"(?<=<h1>).+?(?=</h1>)") matches = re.findall(regex, randStr) for i in matches: print(i) import re # ---------- Negative Look Ahead & Behind ---------- # These are used to look for text that doesn't match # the pattern # (?!expression) : Negative Look Ahead # (?<!expression) : Negative Look Behind randStr = "8 Apples $3, 1 Bread $1, 1 Cereal $4" # Grab the total number of grocery items by ignoring the $ regex = re.compile(r"(?<!\$)\d+") matches = re.findall(regex, randStr) print(len(matches)) # Convert from a string list to an int list matches = [int(i) for i in matches] from functools import reduce # Sum the items in the list with reduce print("Total Items {}".format(reduce((lambda x, y: x + y), matches))) |
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