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Regular Expressions
Regular Expressions (sometimes shortened to regexp, regex, or re) are a tool for matching patterns in text. In Python, we have the re module. The applications for regular expressions are wide-spread, but they are fairly complex, so when contemplating using a regex for a certain task, think about alternatives, and come to regexes as a last resort.
An example regex is r"^(From|To|Cc).*[email protected]" Now for an
explanation:
the caret ^ matches text at the beginning of a line. The following
group, the part with (From|To|Cc) means that the line has to start with
one of the words that are separated by the pipe |. That is called
the OR operator, and the regex will match if the line starts with any
of the words in the group. The .*? means to un-greedily match any
number of characters, except the newline \n character. The un-greedy
part means to match as few repetitions as possible. The . character
means any non-newline character, the * means to repeat 0 or more
times, and the ? character makes it un-greedy.
So, the following lines would be matched by that regex:
From: [email protected]
To: !asp]<,. [email protected]
A complete reference for the re syntax is available at the python docs.
As an example of a "proper" email-matching regex (like the one in the exercise), see this
# Example:
import re
pattern = re.compile(r"\[(on|off)\]") # Slight optimization
print(re.search(pattern, "Mono: Playback 65 [75%] [-16.50dB] [on]"))
# Returns a Match object!
print(re.search(pattern, "Nada...:-("))
# Doesn't return anything.
# End Example
# Exercise: make a regular expression that will match an email
def test_email(your_pattern):
pattern = re.compile(your_pattern)
emails = ["[email protected]", "[email protected]", "wha.t.`1an?ug{}[email protected]"]
for email in emails:
if not re.match(pattern, email):
print("You failed to match %s" % (email))
elif not your_pattern:
print("Forgot to enter a pattern!")
else:
print("Pass")
pattern = r"" # Your pattern here!
test_email(pattern)
# Exercise: make a regular expression that will match an email
import re
def test_email(your_pattern):
pattern = re.compile(your_pattern)
emails = ["[email protected]", "[email protected]", "wha.t.`1an?ug{}[email protected]"]
for email in emails:
if not re.match(pattern, email):
print("You failed to match %s" % (email))
elif not your_pattern:
print("Forgot to enter a pattern!")
else:
print("Pass")
# Your pattern here!
pattern = r"\"?([-a-zA-Z0-9.`?{}]+@\w+\.\w+)\"?"
test_email(pattern)
test_output_contains("Pass")
success_msg("Great work!")
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