12.2. Character Matching in Regular Expressions¶
There are a number of other special characters that let us build even more powerful regular expressions. The most commonly used special character is the period or full stop, which matches any character.
In the following example, the regular expression
F..m: would match any
of the strings “From:”, “Fxxm:”, “F12m:”, or “F!@m:” since the period
characters in the regular expression match any character.
This code searches for lines that start with ‘F’, follow by 2 characters, followed by ‘m:’
This is particularly powerful when combined with the ability to indicate
that a character can be repeated any number of times using the
+ characters in your regular expression. These special characters mean
that instead of matching a single character in the search string, they
match zero-or-more characters (in the case of the asterisk) or
one-or-more of the characters (in the case of the plus sign).
We can further narrow down the lines that we match using a repeated wild card character in the following example:
This code searches for lines that start with ‘From:’ and have an ‘@’ symbol.
The search string
^From:.+@ will successfully match lines that start
with “From:”, followed by one or more characters (
.+), followed by an
at-sign. So this will match the following line:
You can think of the
.+ wildcard as expanding to match all the
characters between the colon character and the at-sign.
It is good to think of the plus and asterisk characters as “pushy” or “greedy”. For
example, the following string would match the last at-sign in the string
.+ pushes outwards, as shown below:
From: firstname.lastname@example.org, email@example.com, and cwen @iupui.edu
It is possible to tell an asterisk or plus sign not to be so “greedy” by adding another character. See the detailed documentation for information on turning off the greedy behavior.