programming-for-dummies

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Finding Strings with Regular Expressions
Finding Strings with Regular Expressions
Before you can manipulate a string, you first must find it. Although some programming
languages include string searching functions, most of them are
fairly limited to finding exact matches of strings.
To remedy this problem, many programming languages (such as Perl and
Tcl) use regular expressions. (A regular expression is just a series of symbols
that tell the computer how to find a specific pattern in a string.)
If a programming language doesn’t offer built-in support for regular expressions,
many programmers have written subprogram libraries that let you
add regular expressions to your program. By using regular expressions, your
programs can perform more sophisticated text searching than any built-in
string functions could ever do.
Pattern matching with the single
character (.) wildcard
The simplest way to search for a pattern is to look for a single character. For
example, you might want to know if a certain string begins with the letter b,
ends with the letter t, and contains exactly one character between. Although
you could repetitively check every three-character string that begins with b
and ends with t, like bat or but, it’s much easier to use a single-character
wildcard instead, which is a dot or period character (.).
So if you want to find every three-letter string that begins with a b and ends
with a t, you’d use this regular expression:
b.t
To search for multiple characters, use the (.) wildcard multiple times to
match multiple characters. So the pattern b..t matches the strings boot and
boat with the two (..) wildcards representing the two characters between
the b and the t.
Of course, the b..t pattern doesn’t match bat because bat has only one
character between the b and the t. Nor does it match boost because boost
has more than two characters between the b and the t.
When using the (.) wildcard, you must know the exact number of characters
to match.