Regular expressions are case sensitive; lower case /s/ is distinct from upper case /S/. We can solve this problem with the use of the square braces [ and ]: /[wW]oodchuck/
/[abc]/ : ‘a’, ‘b’, or ‘c’
/[1234567890]/: any digit
/[2-5]/ 2, 3, 4, or 5
/[A-Z]/ an upper case letter
/[a-z]/ a lower case letter
/[0-9]/ a single digit
The square braces can also be used to specify what a single character cannot be,
by use of the caret ^.
/[^A-Z]/ not an upper case letter
/[^Ss]/ neither ‘S’ nor ‘s’
/[^\.]/ not a period
/[e^]/ either ‘e’ or ‘^’ “look up^now”
/a^b/ the pattern ‘a^b’ “look up a^b now”
/?/, which means “the preceding character or nothing”. We can think of the question mark as meaning “zero or one instances of the
previous character”.
/woodchucks?/ woodchuck or woodchucks
/colou?r/ color or colour
The Kleene star * (pronounced “cleany star”) means “zero or more occurrences of the immediately previous character or regular expression”. So /a*/ means “any string of zero or more a's”. This will match a or aaaaaa.
So the regular expression for matching one or more a is /aa*/, meaning one a followed by zero or more a's.
So /[ab]*/ means “zero or more a’s or b’s”. This will match strings like aaaa or ababab or bbbb.
An integer (a string of digits) is thus /[0-9][0-9]*/ and not /[0-9]*/.
Kleene +, which means “one or more occurrences of the immediately preceding character or regular expression”. Thus, the expression /[0-9]+/ is the normal way to specify “a sequence of digits”.
One very important special character is the period (/./), a wildcard expression
that matches any single character (except a carriage return).
/beg.n/ any character between beg and n begin, beg’n, begun
Anchors are special characters that anchor regular expressions to particular places
in a string. The most common anchors are the caret ^ and the dollar sign $.
The caret ^ matches the start of a line. The pattern /^The/ matches the word The only at the start of a line. Thus, the caret ^ has three uses: to match the start of a line, to indicate
a negation inside of square brackets, and just to mean a caret.
The dollar sign $ matches the end of a line.
/^The dog\.$/ matches a line that contains only the phrase The dog.
\b matches a word boundary, and \B matches a non-boundary. Thus, /\bthe\b/ matches the word the but not the word other.
/\b99\b/ 9 and not 299. But it will match 99 in
disjunction operator, also called the pipe symbol |.
So the pattern /gupp(y|ies)/ would specify that we meant the disjunction only to apply to the suffixes y and ies.