Google Python Class Exercise Solutions

Regular expressions

Basic Patterns

The power of regular expressions is that they can specify patterns, not just fixed characters. Here are the most basic patterns which match single chars:

1. a, X, 9, < -- ordinary characters just match themselves exactly. The meta-characters which do not match themselves because they have special meanings are: . ^ $ * + ? { [ ] \ | ( ) (details below)
2. . (a period) -- matches any single character except newline '\n'
3. \w -- (lowercase w) matches a "word" character: a letter or digit or underbar [a-zA-Z0-9_]. Note that although "word" is the mnemonic for this, it only matches a single word char, not a whole word. \W (upper case W) matches any non-word character.
4. \b -- boundary between word and non-word
5. \s -- (lowercase s) matches a single whitespace character -- space, newline, return, tab, form [ \n\r\t\f]. \S (upper case S) matches any non-whitespace character.
6. \t, \n, \r -- tab, newline, return
7. \d -- decimal digit [0-9] (some older regex utilities do not support \d, but they all support \w and \s)
8. ^ = start, $ = end -- match the start or end of the string
9. \ -- inhibit the "specialness" of a character. So, for example, use . to match a period or \ to match a slash. If you are unsure if a character has special meaning, such as '@', you can put a slash in front of it, @, to make sure it is treated just as a character.