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Regex

· 4 min read
  • Denoted by []
  • [ae] -> matches an 'a' and an 'e', not multiple:
    • i.e. gr[ae]y will match grey and gray
  • [a-z] -> hyphen specifies a range of characters:
    • i.e. [0-9] matches a single digit
    • i.e.2 [a-f] matches a char in a-f
  • [a-z0-9A-F] -> groups of hyphen specifies a group of ranges
  • qx -> caret after openning bracket negates the character class:
    • i.e. qX matches qu in question
    • i.e. must write it at the front of a class/set

Shorthand Character Classes​

  • \d matches a single digit == [0-9]
  • \w mathces a "work character" (alphanumeric characters plus underscore)
  • \s matches a whitespace character (includes tabs and line breaks)

the shorthand character class is software depended

Non-Printable Characters​

  • \t == tab
  • \r == carriage return
  • \n == new line
  • \a == bell
  • \e == escape
  • \f == form feed
  • \v == vertical tab
  • \r\n == windows new line
  • Unicode:
    • \uFFFF or \x{FFFF}

Dot​

  • . matches any char, except line break (depends on software and mode - single line mode includes line breaks)

Anchors

  • ^ matches at the start of the string / line break:
    • ^g for gary will match g
  • $ matches at the end of the string / line break:
    • y$ for gary will match y
  • \b matches at a position that is called a "word boundary". This match is zero-length:
    • \b5\b will match 5 in 5 555 55

Alternation

  • | matches cat and dog in About cats and dogs
  • (|) can group multiple:
    • in cat food or dog food, (cat|dog) food will match both cat food and dog food

Repetition

  • ? means the preceding token in the regular expression optional:
    • for carl and carol, caro?l will match both carl and carol
  • * means the preceding token in the regular expression appears 0 or more
    • for carl, carol and carool, caro*l will match both carl, carol and carool
  • + means the preceding token in the regular expression appears 1 or more
    • for carl, carol and carool, caro+l will match both carol and carool
    • usecase:
    • Matching HTML tags without any attributes:
    • <[A-Za-z0-9]+> is a bad implementation, becase <1> will be matched
    • <[A-Za-z][A-Za-z0-9]*> is a good implemenation
  • {} specifies a specific amount of repetition:
    • \b[1-9][0-9]{3}\b matches a number between 1000 and 9999
    • \b[1-9][0-9]{2,4}\b matches a number between 100 and 99999

Greedy/Lazy Quantifiers ✨​

  • repetition operators or quantifiers are greedy, they will expand as much as possible in general:
    • <.+> matches <html> apsdpdapdapjaid </html> in blablabla <html> apsdpdapdapjaid <html> balabla
  • but if they need to satisfy the remainder of the regex, they will expand to a certain depth:
    • <.+?> matches <html> and </html> in blablabla <html> apsdpdapdapjaid <html> balabla
  • another solution, use negation:
    • <[^<>]+> will do the same job

Backreferences:

  • \1 refers to first capturing group:
    • ([abc])=\1 can match a=a,b=b,c=c
  • \2 \3 are all referring to the indexed capturing group

Name groups:​

  • use (?<groupname>) and \k<groupname> to call and refer previously declared groups

Lookaround:

  • q(?=u) is positive lookahead:
    • it matches the q in question, but not in Iraq.
  • q(?!u) is negative look ahead:
    • it matches the q in Iraq but not in questions
  • (?<=a)b is positive look backwards:
    • it matches the b in abc
  • (?<!a)b is negative look backwards:
    • it doesn't match the b in abc, but matches the b in cbc
    • Best example: how to include whole directory except ./node_modules/
      • ^(?!.*node_modules).*.js
        • ^ means starting of the line
        • (?! -> negative look ahead (from the start of the line
        • .*node_mudoles -> match all files that has node_modules in thier directory string
        • .*.js all js files

Mac Specific

  • Mac Terminal uses BRE link
    • must add \ in front of brackets
    • ! doesn't support lookaround link