regex_training

• based on https://www.udemy.com/course/learn-and-understand-regular-expression-with-examples
• online regex tool: https://regex101.com/

Intro

• regex is a search/find pattern
• Metacharacters are the building blocks of regular expressions(\d, \w, \W etc)
• character classes - matching one or several characters
• quantifiers - number of repetition

Metacharacters

• ^ start of line

• $ end of line

• . period

• • range of elements in []

• [] character class to match anything inside brackets

• [^] negate character class to match string that do not follow the pattern

• \char escape metacharacter - is used to escape metacharacter in string

• '*' - 0 or more time

• • one or more times

• {min, max} - min required, max allowed

• () groups patterns together

• • /pattern/ -> pattern goes inside double slashes

  Exammples

  • [a-z] match from a to z lowercase
  • [A-Z] match from a to z uppercase
  • [0-9] from 0 to 9
  • [a-zA-Z] - upper and lower case letter
  • [a-zA-Z0-9] alphanumerical match
  • [a-z]* zero or many times a-z
  • [a-z]+ one or many times a-z
  • .* match zero or many times from any
  • (?=) positive look ahead
  • [a-z]{1,2} match one or two from a to z
  • [a-z]{1,}match one or unlimited from a to z
  • /pattern/i - case sensitive search

literal characters

• Any character except [^$.|?*+()
• example '/A/' matches word 'And' -> returns true
• example '/A/' doesn't match word 'and' -> return false

Mode or Flag modifiers

• '/.../i' case non sensitive, e.g. '/pattern/i' matches 'Pattern'
• /.../g global, The g modifier is used to perform a global match (find all matches rather than stopping after the first match). e.g. '/a/g' finds all 'aaaaaaa'
• while '/a/' finds only first a in 'aaaaaaaa'

Dot metacharacter

• examples:
  '/./' points to first symbol h in 'hello world'
  '/./' equals to '/[...]/'

Anchors

• '^' caret - matches the very beggining of the string. example, '/^h/' points to fist symbol in 'hello'
• '$' - matches end of the string. example '/o$/' points to last symbol in 'hello'

Escaping metacharacters

• uses \ to escape a character. example '/^^/' matches symbol ^ in string '^hello' or '/$$/' matches to last symbol in 'hello$'

Word boundary

• syntax \bword\b
• example '/\bhello\b/'matches only word hello(the second one) in 'hellohello hello world'