Regular Expressions What is this line all about? while (!($search =~ /^\s*$/)) { It’s a string search just like before, but with a huge twist – regular expression search ^\s*$ is a regular expression that says “look for a line with nothing but white space” – Whitespace: space ( ), tab (\t), formfeed (\f), newline (\n), carriage return (\r)

Regular Expressions A “convenient” way to describe patterns of characters – Characters include “printable” and “meta” characters Three primary concepts : – Concatenation – adjacent characters in the search string must be adjacent in the data string – Alternation – specify a choice of characters that match in a specified position – Repetition – specify how many of a given character must match

Concatenation if ($data =~ /abcdef/) { … } The pattern “abcdef” must show in that order within the variable $data

Alternation if ($data =~ /a(b|c|d|e)f/) { … } The pattern “a(b|c|d|e)f” must be an ‘a’ followed by one of ‘b’, ‘c’, ‘d’, ‘e’, followed by a ‘f’ within the variable $data

Repetition if ($data =~ /ab*f/) { … } The pattern “ab*f” must be an ‘a’ followed by zero or more ‘b’, followed by a ‘f’ within the variable $data * – zero or more instances of the previous character + – one or more instances of the previous character {n} – exactly n instances of the previous character {m,n} – m or m+1, …, n instances of the previous character {n,} – n or more instances of the previous character ? – zero or one instances of the previous character

Meta-characters Anything following a \ Alternation (choice) | Grouping within ( and ) Character classes within [ and ] – e.g. [A-Za-z] all upper and lower case letters – e.g. [abc] a or b or c – same as (a|b|c) – e.g. [^0-9] anything that is not a digit 0 thru 9 Match any –. (the dot) matches all characters. e.g. [.*] zero or more of any character

Meta-characters Beginning and end of a string – ^ what follows must start the string – $ what follows must end the string – /^ matches the ^ – /$ matches the $

Character Classes Use square brackets to denote classes (sets) of characters to be matched [A-Z] match any single uppercase letter [a-z] match any single lower case letter [0-9] match any digit [A-Za-z0-9] match any single letter or digit [^0-9] match any single character that is NOT a digit Note that there is no spaces in the classes (unless you want to match a space)

Matching String matching assumes the longest possible string to formulate the match e.g. “hear ye hear ye” =~ /hear.*ye/ matches the entire string If you want the minimal string you must do the following e.g. “hear ye hear ye” =~ /hear.*?ye/ matches only the first “hear ye”