8 CHAPTER 2 • REGULAR EXPRESSIONS, TEXT NORMALIZATION, EDIT DISTANCE
in a string. The most common anchors are the caret ˆ and the dollar sign $. The caret
ˆ matches the start of a line. The pattern /ˆThe/ matches the word The only at the
start of a line. Thus, the caret ˆ has three uses: to match the start of a line, to in-
dicate a negation inside of square brackets, and just to mean a caret. (What are the
contexts that allow grep or Python to know which function a given caret is supposed
to have?) The dollar sign $ matches the end of a line. So the pattern $ is a useful
pattern for matching a space at the end of a line, and /ˆThe dog\.$/ matches a
line that contains only the phrase The dog. (We have to use the backslash here since
we want the . to mean “period” and not the wildcard.)
Regex Match
ˆ start of line
$ end of line
\b word boundary
\B non-word boundary
Figure 2.7 Anchors in regular expressions.
There are also two other anchors: \b matches a word boundary, and \B matches
a non-boundary. Thus, /\bthe\b/ matches the word the but not the word other.
More technically, a “word” for the purposes of a regular expression is defined as any
sequence of digits, underscores, or letters; this is based on the definition of “words”
in programming languages. For example, /\b99\b/ will match the string 99 in
There are 99 bottles of beer on the wall (because 99 follows a space) but not 99 in
There are 299 bottles of beer on the wall (since 99 follows a number). But it will
match 99 in $99 (since 99 follows a dollar sign ($), which is not a digit, underscore,
or letter).
2.1.2 Disjunction, Grouping, and Precedence
Suppose we need to search for texts about pets; perhaps we are particularly interested
in cats and dogs. In such a case, we might want to search for either the string cat or
the string dog. Since we can’t use the square brackets to search for “cat or dog” (why
can’t we say /[catdog]/?), we need a new operator, the disjunction operator, also
disjunction
called the pipe symbol |. The pattern /cat|dog/ matches either the string cat or
the string dog.
Sometimes we need to use this disjunction operator in the midst of a larger se-
quence. For example, suppose I want to search for information about pet fish for
my cousin David. How can I specify both guppy and guppies? We cannot simply
say /guppy|ies/, because that would match only the strings guppy and ies. This
is because sequences like guppy take precedence over the disjunction operator |.
precedence
To make the disjunction operator apply only to a specific pattern, we need to use the
parenthesis operators ( and ). Enclosing a pattern in parentheses makes it act like
a single character for the purposes of neighboring operators like the pipe | and the
Kleene*. So the pattern /gupp(y|ies)/ would specify that we meant the disjunc-
tion only to apply to the suffixes y and ies.
The parenthesis operator ( is also useful when we are using counters like the
Kleene*. Unlike the | operator, the Kleene* operator applies by default only to
a single character, not to a whole sequence. Suppose we want to match repeated
instances of a string. Perhaps we have a line that has column labels of the form
Column 1 Column 2 Column 3. The expression /Column [0-9]+ */ will not
match any number of columns; instead, it will match a single column followed by
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