Learn regular expressions

Intro

• Regular Expressions = "regex" or "regexp"
• Comes from the ed editor but you'll mostly encounter the grep program
• A quick way of describing a particular pattern of characters in text
• Allows for extremely effective search and replace
• Comes from the ed editor from UNIX but you'll mostly encounter the grep program

Knowing [regular expressions] can mean the difference between solving a problem in 3 steps and solving it in 3,000 steps. When you’re a nerd, you forget that the problems you solve with a couple keystrokes can take other people days of tedious, error-prone work to slog through.

– Cory Doctorow in Here's what ICT should really teach kids: how to do regular expressions

Notes

BRE vs ERE

• grep uses so called "basic regular expressions" (BRE) by default.
• What we normally consider "regular expressions" are actually "extended regular expressions" (ERE)
• The main difference is in backslashing the meta characters:
  • BRE: \?, \+, \{, \}
  • ERE: ?, +, {, }
• EREs can be turned on in grep by passing the -E parameter.
• More details here

Links

• The origins of grep: Brian Kernighan, one of the forefathers of UNIX discusses how grep came to be, and it makes for a rather interesting story! (If you are in a hurry, here is a 10 minute video.)
• RegexOne: interactive tutorial.
• Regex101: an integrated development environment (IDE) for regular expressions.
• Regexper: visualises regular expressions as "proto programs".
• Games:
  • RegexGolf
  • Regex Tuesday
  • RegexCrossword
• There are also some alternatives to grep, esp. ripgrep = rg
• You can even use regex to search prime numbers. Not so easy for email addresses though.

Tips

• Use non-capturing groups, if needed
• If you are looking through file, use find [path] -name [pattern] or even find [path] -regex [pattern] (typically with -type f just for files) instead of grep