Regular expression allow you to specify a pattern of text to search for.
Regular expressions are sort of a simplified mini language for specifying text pattern.
One example of text pattern is a phone number.
For US and Canada the phone number is like a following
414-555-0000

The first 3 digits are area code.

We as a human know that the above number is phone number while 414,555,0000 is not.

In [1]:
def isPhoneNumber(text):      #determining if the phone number is us, canadian style or not.
    if len(text) != 12:
        print("not 12 digits")
        return False          #not phone number sized

    for i in range(0,3):
        if not text[i].isdecimal():      #if not a decimal/number character we can use 
            print("first 3 characters are not numbers")
            return False

    if text[3] != "-":
        print("missing dash after the area code")
        return False       #missing dash

    for i in range(4,7):
        if not text[i].isdecimal():
            print("three characters in the middle are not numbers")
            return False

    if text[7] != "-":
        print("missing second dash")
        return False

    for i in range(8,12):
        if not text[i].isdecimal():
            print("last four characters are not numbers")
            return False      #missing the last 4 digits

    return True

Take a note that a function does not execute anything after it encounters a return statement.

In [2]:
isPhoneNumber("414-555-0000")
Out[2]:
True
In [3]:
isPhoneNumber("ldskfjksdjfl")

first 3 characters are not numbers
Out[3]:
False
In [4]:
isPhoneNumber("4149555-0000")

missing dash after the area code
Out[4]:
False
In [5]:
isPhoneNumber("414-555-000")

not 12 digits
Out[5]:
False
In [6]:
isPhoneNumber("414-555-00a0")

last four characters are not numbers
Out[6]:
False


But what if we have a message and want to find out if the phone number exists in that message
We do that the following way

First we will redefine our function so that it returns only true or false and does not print any message at all on the screen.

In [7]:
def isPhoneNumber(text):
    if len(text) != 12:
        return False

    for i in range(0,3):
        if not text[i].isdecimal():
            return False

    if text[3] != "-":
        return False

    for i in range(4,7):
        if not text[i].isdecimal():
            return False

    if text[7] != "-":
        return False

    for i in range(8,12):
        if not text[i].isdecimal():
            return False

    return True

Now our number finding function.

In [8]:
def numberFinder(message):
    foundNumber = False
    for i in range(len(message)):
        chunk = message[i:i+12]
        if isPhoneNumber(chunk):
            print("The number is " + chunk)
            foundNumber = True
    if foundNumber == False:
        print("no numbers found in the message")
    return foundNumber
In [9]:
numberFinder("Hey, you can call me on 414-444-5757 or if that number is not reachable you can use my other number 423-333-5555.")

The number is 414-444-5757
The number is 423-333-5555
Out[9]:
True
In [10]:
numberFinder("hi, 433-4343-4343")

no numbers found in the message
Out[10]:
False
In [11]:
numberFinder("423-333-3333")

The number is 423-333-3333
Out[11]:
True
In [12]:
someMessage = "hello my number is 433-433-4343. Oops not that one, my real number is 333-444-2222. Ok bye now"
In [13]:
numberFinder(someMessage)

The number is 433-433-4343
The number is 333-444-2222
Out[13]:
True

THAT IS A LOT OF CODE!!

re module

In [14]:
import re

Regex strings oftern use \ backslashes (like \d), so they are often raw strings: r'\d'

Calling the re.compile() function to create a regex object.
\d is the regex for numeric digit character.

In [15]:
phoneNumRegex = re.compile(r'\d\d\d-\d\d\d-\d\d\d\d')

Call the regex object's search() method to create the match object

In [16]:
someMessage = "hello my number is 433-433-4343. Oops not that one, my real number is 333-444-2222. Ok bye now"
In [17]:
matchObjBlaBla = phoneNumRegex.search(someMessage)

match objects have a method called group()

In [18]:
print(matchObjBlaBla.group())     #to just print out the found element.

433-433-4343
In [19]:
print(matchObjBlaBla)

<_sre.SRE_Match object; span=(19, 31), match='433-433-4343'>

It gave out only one occurance, for all occurences, we use findall() method on regex object.

In [20]:
matchObjBlaBla = phoneNumRegex.findall(someMessage)   #this will return a list value
In [21]:
print(matchObjBlaBla.group())

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-21-1d06ddbf6fe9> in <module>()
----> 1 print(matchObjBlaBla.group())

AttributeError: 'list' object has no attribute 'group'
In [22]:
print(matchObjBlaBla)

['433-433-4343', '333-444-2222']