jurafsky&martin_3rdEd_17 (1).pdf

# It is often useful to be able to refer to a

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It is often useful to be able to refer to a particular subpart of the string matching the first pattern. For example, suppose we wanted to put angle brackets around all integers in a text, for example, changing the 35 boxes to the < 35 > boxes . We’d like a way to refer to the integer we’ve found so that we can easily add the brackets. To do this, we put parentheses ( and ) around the first pattern and use the number operator \1 in the second pattern to refer back. Here’s how it looks: s/([0-9]+)/<\1>/ The parenthesis and number operators can also specify that a certain string or expression must occur twice in the text. For example, suppose we are looking for the pattern “the Xer they were, the Xer they will be”, where we want to constrain the two X’s to be the same string. We do this by surrounding the first X with the parenthesis operator, and replacing the second X with the number operator \1 , as follows: /the (.*)er they were, the \1er they will be/

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18 C HAPTER 2 R EGULAR E XPRESSIONS , T EXT N ORMALIZATION , E DIT D ISTANCE Here the \1 will be replaced by whatever string matched the first item in paren- theses. So this will match The bigger they were, the bigger they will be but not The bigger they were, the faster they will be . This use of parentheses to store a pattern in memory is called a capture group . capture group Every time a capture group is used (i.e., parentheses surround a pattern), the re- sulting match is stored in a numbered register . If you match two different sets of register parentheses, \2 means whatever matched the second capture group. Thus /the (.*)er they (.*), the \1er we \2/ will match The faster they ran, the faster we ran but not The faster they ran, the faster we ate . Similarly, the third capture group is stored in \3 , the fourth is \4 , and so on. Parentheses thus have a double function in regular expressions; they are used to group terms for specifying the order in which operators should apply, and they are used to capture something in a register. Occasionally we might want to use parenthe- ses for grouping, but don’t want to capture the resulting pattern in a register. In that case we use a non-capturing group , which is specified by putting the commands non-capturing group ?: after the open paren, in the form (?: pattern ) . /(?:some|a few) (people|cats) like some \1/ will match some cats like some people but not some people like some a few . Substitutions and capture groups are very useful in implementing simple chat- bots like ELIZA (Weizenbaum, 1966) . Recall that ELIZA simulates a Rogerian psychologist by carrying on conversations like the following: User 1 : Men are all alike. ELIZA 1 : IN WHAT WAY User 2 : They’re always bugging us about something or other. ELIZA 2 : CAN YOU THINK OF A SPECIFIC EXAMPLE User 3 : Well, my boyfriend made me come here.
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