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Regular Expressions (regex)
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Regular Expressions (regex) are a sequence of characters that define a search pattern.

Grep is a command-line utility for searching plain-text data sets for lines that match a regular expression.  
Its name comes from the ed command g/re/p (globally search a regular expression and print), which has the same effect:  
doing a global search with the regular expression and printing all matching lines.  

- `\` - escape character (e.g. \. - match a dot character)
- `$` - end of line (e.g. a$ - lines that end with a)
- `^` - start of line (e.g. ^a - lines that start with a)
- `.` - any character (e.g. a.b - lines that contain a, then any character, then b)