Regular Expression

programming_language

basic token

• match any single character
  .
• match a former character
  
  • {n} exactly n times
    {n,} at least n times
    {n,m} at least n times, at most m times
  • ? 0 or once = {0,1}
    * any times = {0,}
    + once or more = {1,}
• use \ to convert
  For instance, \* matches a *.
  (, - need to be converted to match.

• use ? after + or * to prevent greedness
  .* matches as much as possible
  .*? prevents this

• use square bracket [] to sepcify some characters
  Eg. [Aa]pple matches both "Apple" and "apple".

• use - to show a range of alphabets or numbers
  [0-9] specifies any single number.
  [a-zA-Z] specifies any alphabet.

• use ^ before a group of characters to match any character except them
  Eg. appl[^e] matches "applw" and "appl" except "apple".

• Another use of \:
  Transform a single character into a group

  • \s matches any space, including SPACE, TAB, NEXTLINE.
  • \t matches any non-space.
  • \b matches the boundary of a word.
    Eg. er\b matches "number" not "verb".
  • \B doees not match the boundary of a word
    Exactly the reverse of the example above.
  • \d matches a digit = [0-9]
  • \D matches a non-digit = [^0-9]

• Grouping
  To make a sub-pattern of a pattern, grouping begins with ?:.
  Eg. (?:ab){1,2} matches both "abab" and "ab".

• tube character

  • | matches left OR right
    Eg. a(?:b|c)d matches both "abd" and "acd".

• Capturing group
  extract what you group using ()
  Eg. match "aab123dd" with ab(\d{3})d to extract "123".

• The beginning of a line ^.
  The ending of a line $.

• Change some default settings

  • Add g to do a global match /regular expression/g.
  • ignore case /regular expression/i