Regex

Question 1

Square Brackets

Answer 1

• If we put several characters inside square brackets– […]– this means a choice between characters: str.matches(“[Tt]rue”)

Question 2

Pipe Character

Answer 2

• When we want to match alternatives for a whole string, we instead put a pipe character– |– between the alternatives: str.matches(“[Tt]rue|[Yy]es”)

Question 3

Character Ranges

Answer 3

• To match any lower case letter, we can write:[a-z] Similarly, to match a digit, we can write:[0-9]
• We can combine single characters and ranges, and/or combine multiple ranges: [a-zA-Z] This matches a lower or upper case letter in the range A-Z.

Question 4

Negation

Answer 4

• To say “not in the range…”, we put a hat symbol ^ at the beginning of the character class expression. To say “not a digit”, we can write:[^0-9]

Question 5

Intersection &&

Answer 5

• Intersection means “in this class AND in this one”. It is really useful when we combine an intersection with a negation to say “in this class BUT NOT in this one”.
• [0-9&&[^5]] This says a digit except 5
• [a-z&&[^aeiouy]] This says any lower case letter except those representing vowels.

Question 6

Dot .

Answer 6

• The dot essentially matches any character. The following expression means “a digit plus any other character”:[0-9].

Question 7

Repetition

Answer 7

• Repetition operators are placed ==after== the item to be repeated. The following operators all behave in a similar way: they are placed after the repeated item.
• *: means zero or more repetitions: for example: [0-9]* means any number of digits
• ?: means zero or one repetitions: for example: [0-9]? means an optional digit
• +: means one or more repetitions: for example: [0-9]+ means one or more digits
• {x,y}: means between x and y instances of..: for example: [0-9]{10} means ten digits: .{3} mean three instances of any character
• {x,}: mean at least x instances of…: for example: .{5,} mean at least 5 characters

Question 8

Named Character Classes

Answer 8

• For certain common character choices we can put a backslash followed by a character class name.
• To match a single digit, we can write the expression \d.
• So we can now write our ‘has ten characters’ method as follows: str.matches(“.*\d{10}.*”)
• Another useful character class is \s. This matches so-called whitespace: spaces, tabs and line breaks.
• To write \s inside a string literal, we need to double the backslash: “\s”.

Question 9

Pattern() method

Answer 9

• We call the static method Pattern.compile(), passing in the expression. This method returns a Pattern object. Hanging off this object is an internal representation of the pattern in a form that makes it efficient to perform matches.
• Pattern patt = Pattern.compile(“.*?[0-9]{10}.*”);*

Question 10

Matcher() method

Answer 10

• To check whether a particular string matches the pattern, we call the matcher() method on the Pattern object, passing in the string to match:
• Matcher m = patt.matcher(str);*

Question 11

Usage with Strings

Answer 11

• The third step is to call the matches() method on the Matcher we have just created, which returns a boolean indicating whether or not the string passed into the matcher() method matches the regular expression.
  public boolean containsTenDigits(String str) {
  Pattern patt = Pattern.compile(“.*?[0-9]{10}.*”);
  Matcher m = patt.matcher(str);
  return m.matches();
  }

Question 12

Useful Links

Answer 12

• https://www.javamex.com/tutorials/regular_expressions/index.shtml