About regular expressions

The true power of regular expressions — it also includes a nice explanation of what regular means in this context
softwareengineering: Is it a must for every programmer to learn regular expressions?
softwareengineering: When you should NOT use Regular Expressions?
codinghorror: Now You Have Two Problems
wikipedia: Regular expression — this article includes discussion on regular expressions as a formal language as well as details on various implementations
swtch - stuff about regular expression implementation engines

Learning resources

Note that some of these resources are not specific to Python, so use them with caution and check if they apply to Python's syntax and features.

docs.python: Regular Expression HOWTO
stackoverflow: python regex
PythonVerbalExpressions — construct regular expressions with natural language terms
CommonRegex — collection of common regular expressions
Awesome Regex — curated collection of libraries, tools, frameworks and software
Generate strings that match a given regular expression
stackoverflow: regex FAQ
- stackoverflow: regex tag is a good source of exercise questions
rexegg — tutorials, tricks and more
regular-expressions — tutorials and tools
regexcrossword — tutorials and puzzles, only supports JavaScript flavor
regex101 — visual aid and online testing tool for regular expressions, select flavor as Python before use
debuggex — railroad diagrams for regular expressions, select flavor as Python before use