Understanding Regular Expressions - HTML, CSS, and Dynamic HTML - page 1157

Figure H-1 Character positions

Regular
Expression

Character Description Text String Match?

^ start of text string /^audio/ audio studio yes

great audio no

$ end of text string /audio$/ audio studio no

great audio yes

\b word boundary /audio\b/ audio studio yes

audiophile no

\B no word boundary /audio\B/ audio studio no

audiophile yes

Regular expressions recognize the beginning and end of a text string, indicated by the
^ and $ characters, respectively. The following pattern uses the ^ character to mark the
start of the text string:

/^audio/
Any text string that starts with the characters audio is matched by this regular expression;
however, a text string such as great audio , in which the characters audio are not at the
start of the text string, would not be matched. In the same way, the end of the text string
is indicated by the $ character and thus, the following regular expression matches only
text strings that end with audio :

/audio$/
The ^ and $ characters are often used together to define a pattern for a complete text string.
For example, the following pattern matches only the text string audio and nothing else:

/^audio$/
The other positioning characters are used to locate words within a text string. The term
word has a special meaning in regular expressions. Words are composed of word char-
acters, where a word character is any letter or number, or the underscore. Symbols such
as *, &, and - are not considered word characters, nor are spaces, periods, or tabs. Thus
R2D2 is considered a single word, but R2D2&C3PO is considered two words, with the &
symbol acting as a boundary between the words. In a regular expression, the presence of
a word boundary is indicated with the \b symbol. Thus, the following pattern matches
any word that starts with the characters art , but does not match words that do not start
with these characters, such as dart :

/\bart/
The \b symbol can also indicate a word boundary at the end of a word. Thus, the follow-
ing regular expression matches words such as dart and heart , but not artist :

/art\b/
If you place the \b symbol at both the beginning and the end of a pattern, you define
a complete word. Thus, the following expression matches only the word art but no
other word:

/\bart\b/