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

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])