REGular EXpression or regex: String containing a combination of normal characters and special metacharacters that describes patterns to find text or positions within a text
Applications: :String manipulation e.g. replace and andspecific substrings :String formatting e.g. interpolating a string in a template :Basic and advanced regular expressions e.g. nding complex patterns in a string
Why do we need regular expressions ? Clean dataset to prepare it for text mining or sentiment analysis Process email content to feed a machine learning algorithm that decides whether an email is spam Parse and extract speci c data from a website to build a database
String formating includes: Positional formatting Formatted string literals Template method(Use many $identifier)
Which should I use? str.format() :
- Good to start with. Concepts apply to f-strings.
- Compatible with all versions of Python. f-strings:
- Always advisable above all methods.
- Not suitable if not working with modern versions of Python (3.6+). Template strings:
- When working with external or user-provided strings
Type conversions allowed:
- !s (string version)
- !r (string containing a printable representation, i.e. with quotes)
- !a (some as !r but escape the non-ASCII characters)
Standard format specifier:
- e (scientific notation, e.g. 5 10^3)
- d (digit, e.g. 4)
- f (float, e.g. 4.5353)
Escape Sequences:
- backslashes \
- Use $$ to escape the dollar sign
Python Supported Methods Library: re
| re.search(r"regex",string) | re.match(r"regex",string) | re.findall(r"regex",string) |
|---|---|---|
| search everywhere | anchored at beginning | return all matches |
| Metacharacter | Meaning |
|---|---|
| \d | digit |
| \w | word character |
| \W | non-word character |
| \s | whitespace |
| \S | pattern containing no whitespace |
| . | match any character(except newline) |
| ^ | start of the string |
| $ | end of the string |
Qantifiers tells the regex engine how many times to match a character immediately to its left.
| Quantifiers | Meaning |
|---|---|
| + | one or more |
| * | no or more |
| {} | number of times the character on the left appears |
| {n,m} | n times at least , m times at most |
| ? | zero or more |
Two types of matching methods:
- Greedy
- Non-greedy or lazy
| GREEDY | NON-GREEDY |
|---|---|
| 1. Match as | 1. Match as few |
| many characters | characters as |
| as possible; | needed; return |
| return | shortest match |
| longest match | |
| 2. Standard | 2. Append ? to |
| quantifiers are | non-greedy |
| greedy by default | match |
Capturing Groups Match a specific subpattern in a pattern.Groups can be used to choose between optional patterns.
- Alternation - Use non-capturing groups for alternation
- Non-capturing Group - Match but not capture a group.Useful when agroup is not backreferenced;Add ?: : (?:regex)
Numbered Groups
Named Groups Give a name to the groups (?P regex)
Backreferences Using capturing groups to reference back to a group
Looking Around Allow us to con rm that sub-pattern is ahead or behind main pattern
- Look Ahead positive: (?=regex) negative: (?!regex)
- Look Behind positive: (?<=regex) negative: (?<!regex)
For more information and guide to building regex check [link] (https://regex101.com/)
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