Regular Expressions

Connor Sample - https://tabulate.tech

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https://regex101.com

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What are regular expressions?

• Sequences of characters that define a search pattern
• Pattern matching and manipulation of text

Common use cases

• Searching for specific patterns in log files
• Parsing and extracting data from structured text
• Validating input in scripts and programs
• Utilies such as grep, vim, sed, and awk support regex

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Basic Syntax

• Literal characters: /hello/
• Anchors: Define positions within the text
  • /^start/
  • /end$/
• Or: Combine two predicates together with the OR operator
  • /start|end/

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Basic Syntax - Character classes

• /[a-z]/
• /[A-Z0-9?]/
• /[^A-Z]/
• The dot matches any character except line endings (\n, \r)
  • /c.t/
• Escape sequences can be used as shorthands
  • \s
  • \d
  • \t

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Basic Syntax - Quantifiers

• Quantifiers: Specify the number of occurrences
  • /[0-9]{3}/
  • /[0-9]{3,5}/
  • /[A-Za-z]*/
  • /[A-Za-z]+/
  • /[a-z]?/

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Escaping special characters

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Examples

1. Finding patterns in files:
   • Use grep to search for specific strings or patterns
   • grep 'error' logfile.txt
   • grep -E '([0-9]{1,3}\.){3}[0-9]{1,3}' access.log
2. Replacing text in files:
   • Use sed or awk to perform find and replace operations
   • sed -i 's/\.html"/"/g' file.txt
3. Validating input:
   • Ensure input adheres to specific formats or constraints
   • <input type="text" pattern="[A-Za-z]{3}"

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Advanced Techniques

• Grouping and capturing: Use parentheses to capture parts of a match
  • Text: “Name: John, Age: 50”
  • Regex: Name: (\w+), Age: (\d+)
  • Non-capturing groups can be created as such: (?:\w+)
• Lookahead and lookbehind: Specify conditions ahead or behind the match
  • Text: “apple, banana, pear”
  • Regex: \w+(?=,)
  • Text: “apple - banana - pear”
  • Regex: (?<=-) \w+

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Word Boundary Marker

• \b to match the position between word characters and non word characters
• Zero-width assertion
• “this is test” vs “this is testing”

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More Advanced Techniques

• Backreferences: Refer to captured groups
  • Text: “Ray loves Python so so much”
  • Regex: (\b\w+\b) \1
• Greedy vs lazy matching: Control the greediness of quantifiers with ?
  • Text: <div>Super</div> <p>Cool</p>

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Example 1: Matching Digits

Text: “I have 3 apples and 5 oranges.”

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Example 1: Solution

Regex: \d+

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Example 2: Matching Words with 3 characters

\b can be used to match “word-boundaries”, which is the space around word characters (\w).

Text: “The quick brown fox jumps over the lazy dog.”

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Example 2: Solution

Regex: \b\w{3}\b

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Example 3: Matching Parts of a Date

Use capture groups to extract March, 5, and 2024. Text: March 5th, 2024

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Example 3: Solution

Regex: ([A-Za-z]+)\s+(\d{1,2})(?:[A-Za-z]*),\s+(\d{4})

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Example 4: Matching Email Addresses

Text: “Contact us at payment@uugix.org.”

Simplified email specification:

• Any uppercase or lowercase letters; numbers; the following symbols: ._%+-

Domain specification:

• Any letter (case insensitive), numbers, or the hyphen.
• Also support subdomains and TLDs of various lengths

Use \b to ensure the email is standalone

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Example 4: Solution

Regex: \b[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\.[A-Za-z]{2,}\b

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Best practices

• Don’t create an overconvoluted and undocumented mess
• Understand when you should and shouldn’t use regex

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(?:[a-z0-9!#$%&'*+/=?^_`{|}~-]+(?:\.[a-z0-9!#$%&'*+/=?^_`{|}~-]+)*|"(?:
[\x01-\x08\x0b\x0c\x0e-\x1f\x21\x23-\x5b\x5d-\x7f]|\\
[\x01-\x09\x0b\x0c\x0e-\x7f])*")@(?:(?:[a-z0-9](?:[a-z0-9-]*[a-z0-9])?\.)
+[a-z0-9](?:[a-z0-9-]*[a-z0-9])?|\[(?:(?:(2(5[0-5]|[0-4][0-9])|1[0-9][0-9]|
[1-9]?[0-9]))\.){3}(?:(2(5[0-5]|[0-4][0-9])|1[0-9][0-9]|[1-9]?[0-9])|
[a-z0-9-]*[a-z0-9]:(?:[\x01-\x08\x0b\x0c\x0e-\x1f\x21-\x5a\x53-\x7f]|\\
[\x01-\x09\x0b\x0c\x0e-\x7f])+)\])

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import re

pattern = re.compile(r"""
\d+           # match one or more digits
\.            # match the `.` character
[A-Za-z]{5}   # match 5 letters of any case 
""", re.X)

pattern2 = re.compile(r"""
(?x)  # verbose mode
\d*   # match an optional digit
""")

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Q&A/Practice

• Extract the three parts of a phone number with capturing groups
• Parse dash-delimited dates (2024-03-21)
• Write a regex to match URLs
• Extract the parts of markdown links
  • [Display Text](https://tabulate.tech)
• Extract the arguments from a Python function signature:
  • def func(arg1, arg2):