Regular Expressions

Learning Objectives

• Describe regular expressions
• Precompile regular expressions for speed
• Construct regular expressions using literal and special characters
• Group regular expression patterns for more flexible matching
• Write regular expression with repeating patterns
• Write code to extract text
• Write code to substitute text

What are Regular Expressions?

• ways to search, extract and substitute patterns in string
• use literal and special characters
• Act as a compact mini language

Regular Expressions Advantages

• works well with strings with patterns
• more compact code than using string methods , slicing and or indexing easier to maintain Available in other programming languages

String Examples With Patterns

• License plate numbers
• phone numbers 1-500-555-1212 415-555-2020 (650)777-5555
• filenames

The Regular Expression Module

• Built in module for regular expressions
• contains methods to search for regex patterns
• returns a match object
  • Contains details about the matched pattern

Using the Regex Module

import the re module => call search methods with regular expression pattern and string => Match object is returned

Search Method Method

Search Method

re.search(pattern, string)

# return a match object if a regex pattern is found in string 
# return None if not found

Match Method

re.match(pattern, string)

# return a match object if a regex pattern is found at the beginning of string 
# return None if not found

Find All All Method

re.findall(pattern, string)

# return a list of all substrings in string that matches the regex pattern 
# return empty list if nothing is found

Search Example

import re
pattern= 'ware'
s = 'software hardware'
 if re.search(pattern, s):
    print('matched')

Match Object

• returned when a pattern is found
• contains additinal information about a match, such as groups
• group() method returns actual pattern that was matched

Match Object Example

import re
pattern= 'ware'
s = 'software hardware'
mo = re.search(pattern, s)
 if mo:
    print(mo.group())
    # Output ware