26.Regular_Expressions.md

Regular Expressions and Wildcard Characters

Where Regular Expressions are Applicable?

1. In Form Validations / To Build Validation Frameworks

   • To validate mobile numbers, email IDs, employee IDs.

2. To Develop Pattern Matching Applications

   • Examples: ctrl+f for find, grep for search.

3. To Develop Translators like Compilers, Assemblers, Interpreters

   Compiler Design:

   • LA → Lexical Analysis / Tokenization / Scanning.
   • SA → Semantic Analysis.
   • SA → Syntax Analysis.
   • ICG → Intermediate Code Generation.
   • CO → Code Optimization.
   • TCG → Target Code Generation.

4. To Develop Digital Machines

   • Examples: Binary Incrementer, Multiplier, etc.

5. To Develop Communication Protocols

   • Examples: TCP/IP, UDP.

If we want to represent a group of strings according to a particular pattern, then we should go for regular expressions. By using wildcard characters, we can build regular expressions. A wildcard character can be used as a substitute for a required sequence of characters in the regular expression.

1. * → Represents zero or more characters.
2. ? → Represents only one character.
3. [] → Range of characters.
4. [abc] → Either 'a', 'b', or 'c'.
5. [!abc] → Any character except 'a', 'b', and 'c'.
6. [a-z] → Any lowercase alphabet symbol.
7. [A-Z] → Any uppercase alphabet symbol.
8. [a-zA-Z] → Any alphabet symbol.
9. [0-9] → Any digit from 0 to 9.
10. [a-zA-Z0-9] → Any alphanumeric character.
11. [!a-zA-Z0-9] → Any character except alphanumeric (i.e., special symbols).
12. [[:lower:]] → Any lowercase alphabet symbol.
13. [[:upper:]] → Any uppercase alphabet symbol.
14. [[:alpha:]] → Any alphabet symbol.
15. [[:digit:]] → Any digit from 0 to 9.
16. [[:alnum:]] → Any alphanumeric character.
17. [![:digit:]] → Any character except digits.
18. {} → List of files with comma separators.