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regex

Regular expressions are a kind of language within a language, designed to help programmers with their searching task.
in general, a regex search runs from left to right, and once a source's character has been used in match, it can't be reused.

metacharacter:

  • \d  digit
  • \s  white space 
  • \w word character (letter, digit or "_")
  • [a-f] range from a to f 
[a-fA-F]  looking for the occurrences of a-f or A-f not fA combination
  • ^ to negate the character specified
  • nested brackets  to create a union set
  • && to specify intersection of a set
  • quantifiers "allows to specify number of occurrences to match against "
    • +  one or more
    • *  zero or more
    • ?  zero or one

    • greedy: read first the whole string, if no match, move backward character by character till a match is found.
    • reluctant , however, take the opposite approach: They start at the beginning of the input string, then move forward searching for match The last thing to try is the entire input string.
    • moving from greedy to reluctant add ? so find? --> find??


To match a pattern exactly n number of times, simply specify the number inside a set of braces  ex: a{3}
To require a pattern to appear at least n times, add a comma after the number a{3,}

Pattern pattern = Pattern.compile("regex");
Matcher matcher=pattern.matcher("String to look into");
matcher.find() start searching, matcher.group() for retrieving matched string




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