Searching using regular expressions

A regular expression is also a ‘special text string’ for describing a search pattern. Regular expressions define patterns of characters that are applied to a block of text for the purpose of locating that text. ‘Jack and Jill went up the hill to fetch a pale of water’ Regular expression: ‘e’ (single literal character) (The ‘regex’ once told will go onto find further occurrences of ‘e’)

Searching using regular expressions The special text string might take on a different form. ‘The bed had been badly put together, but I bedded down for the night all the same’ Regular expression: ‘{b (a | e) d}’ (register bad & bed) (This kind of a function could be used to search web pages and word documents for important strings)

Searching using regular expressions Simple searching Any single character matches itself, unless it is a metacharacter with a special meaning. Characters that normally function as metacharacters are preceded by a backslash when they need to be Interpreted literally. Example: ‘bed’ matches substring ‘bed’ in ‘bedded

Searching using regular expressions Dot matches almost any character The dot matches a single character without caring what that character is, unless it is the newline character. Example: Therefore,.an matches strings: -can -flan -ban -ran

Searching using regular expressions Predefined shorthand classes In predefined classes, the metacharacter ‘\’ functions to escape the normal meaning of the following characters (eg \d means treat ‘d’ as a metacharacter, not as a literal character. Example: The x \ d matches “x0”, “x1”, “x2”, “x3”, “x4”, “x5” (‘d’ matches a single numeric character)

Searching using regular expressions Predefined shorthand classes Other examples include: Example: The be \ w matches “bed”, “bee”, “beg”, “ben”, “bet” (\w matches a single alphanumeric character (a – z, A – Z, 0 - 9) and the underscore character _. The be \ W matches “be!”, “be”, “be£”, “be$”, “be%” (\W matches a single non-alphanumeric character. \W is short for [ \w]

Searching using regular expressions Predefined shorthand classes Other examples include: Example: /D matches a single non numeric character and is equivalent to [ \d] /s matches a single space. /S matches any single non-space character and is the equivalent of [ \s]