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Appendix A: Regular Expressions

Appendix A. Regular Expressions

The following table lists special characters in regular expressions and their meanings.

Type Description Example
^ Beginning of input or line ^T matches “T” in “This good earth” but not in “Uncle Tom.”
$ End of input or line h$ matches “h” in “teach” but not in “teacher.”
* The preceding character zero or more times um* matches “um” in “rum,” “umm” in “yummy,” and “u” in “huge.”
+ The preceding character one or more times um+ matches “um” in “rum” and “umm” in “yummy.”
? The preceding character at most once st?on matches “son” in “Johnson” and “ston” in “Johnston.”
. Any single character except newline .an matches “ran” and “can.”
x|y Either x or y FF0000|000FF matches FF0000 in BGCOLOR=#FF0000 and 0000FF in FONT COLOR=#0000FF.
{n} Exactly n occurrences of the preceding character o{2} matches “oo” in “loom” and the first two o's in “mooooo.”
{n,m} At least n and at most m occurrences of the preceding character F{2,4} matches FF in #FF0000 and the first four Fs in #FFFFFF.
[abc] Any one of the characters enclosed in the brackets [aeiou] initially matches “a” in “apple,” “e” in “egg,” and “i” in “pig.” Specify a range of characters with a hyphen (for example, [a-f] is equivalent to [abcdef]).
[^abc] Any character not enclosed in the brackets [^aeiou] initially matches “r” in “orange,” “b” in “book,” and “k” in “eek!”. Specify a range of characters with a hyphen (for example, [^a-f] is equivalent to [^abcdef]).
\b A word boundary (such as a space or carriage return) \bb matches “b” in “book” but nothing in “goober” or “snob.”
\B A nonword boundary \Bb matches “b” in “goober” but nothing in “book.”
\d Any digit character. Equivalent to [0-9]. \d matches “3” in “C3PO” and “2” in “apartment 2G.”
\D Any nondigit character. Equivalent to [^0-9]. \D matches “S” in “900S” and “Q” in “Q45.”
\f Form feed  
\n Line feed  
\r Carriage return  
\s Any single whitespace character, including space, tab, form feed, or line feed \sbook matches “book” in “blue book.”
\t A tab  
\w Any alphanumeric character, including underscore. Equivalent to [A-Za-z0-9_]. b\w* matches “barking” in “the barking dog” and both “big” and “black” in “the big black dog.”
\W Any nonalphanumeric character. Equivalent to [^A-Za-z0-9_]. \W matches “&” in “Jake & Mattie” and “%” in “100%”.



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