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RegEx

Crash Course

Why RegEx?

  • Unbelievably Powerful
  • highly reusable and highly portable
  • Solve countless day to day problems
  • Widely Supported

What is RegEx?

A pattern matching language. Yes, a language!

A regex is a string. For instance, foo is a regex. so is [A-Z+:\d+]

There's a beautiful language behind that obfuscated mess of characters.

Simple Expressions

Let us start with basic patterns without weird characters.

abc

a thing, followed by another thing, followed another thing!

Simple Expressions

pattern matches
abc abc,abcd,zabc
123 1234,abc123,za234123bc
a1 a1,a1bc,a12b2c

Meta characters

Characters with special meaning to them.

^ [] () {} . * \ | + ? $ and sometimes -

Match Any Character with .

Matches any character. (Wild card)

s.n matches san, sun, son and etc.

Line Anchors: ^ and $

^ - Start of Line; $ - End of Line

pattern matches
^Hi line starts with Hi
Hello$ line ends with Hello
^san$ line that has only san
^$ Empty line

Character Class: []

match any one of several characters.

gr[ea]y - "g, followed by r, followed by either an e or an a, followed by y."

If you use [^...] instead of [...], the class matches any character that's not in the character class.

Character Class Metacharacter: -

specify range between two characters

pattern matches
[a-c] Any of a b c
[0-9] Any of 0 1 2 3 4 5 6 7 8 9
[A-Za-z] Any of the English alphabet

[012345abcedfABCDEF] is same as [0-5a-fA-F]

Alternation Metacharacter: |

| means either - combines two expressions into one

Mike and Michael are separate, but Mike|Michael is one that matches either.

Matching Optional Items: ?

placed after a character that is allowed, but not required, at a certain position in an expression.

flavou?r - matches flavor (US), flavour (UK)

"f, followed by l, followed by a, followed by v, followed by o, followed by an optional u, followed by r."

Other Quantifiers: + and *

number of times the preceding character can appear in an expression

Character Meaning
? 0 or 1
+ 1 or more
* 0 or more

Interval Quantifier: {}

{min,max} metasequence specifies number of times an item can match

TODO: add example

Escape Character: \

\ escapes metacharacters so you can match those in patterns.

TODO: add example

Paranthesis for Grouping: ()

http://([^/]+) - matches the domain part alone.

http://github.com/sanspace

github.com

[^/]+ is grouped as subportion of the expression

Examples - Username

Requirements

  • Alphanumeric
  • No special characters except underscore
  • Minimum 8 characters, Maximum 16 characters

^[a-zA-Z0-9_]{3,16}$

Match an IP address

169.254.169.254

Not so fast

Some people, when confronted with a problem, think "I know, I'll use regular expressions." Now they have two problems.

Not so fast

not that regular expressions are evil, per se, but that overuse of regular expressions is evil.

Not so fast

  • Know the flavors of different programming languages
  • Know what you are doing
  • RegEx could be slow for complex patterns

References

Credits