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- (Instructor) To finish
off our discussion

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of regular expressions, let's talk

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about how you can capture substrings

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within a matched set of characters.

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So for that purpose,

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I've already put in a few snippets here.

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We've imported the RE module
so we can use its functions.

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We've defined a variable called text,

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which contains the text in
which we will be searching,

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we've defined a regular
expression pattern variable,

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and this is the regular expression

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we're going to search for.

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And you'll notice that we have within

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that expression a couple of
parenthesized subexpressions,

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two of them to be exact,

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and what the parentheses
meta characters mean is that

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if the entire pattern
matches whatever string

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we're searching in, then we want

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to capture just the pieces of that pattern

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that are enclosed in parentheses.

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So this first piece here
is looking for two words

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each of which start with
a capital first letter,

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followed by one more lower case letters,

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separated by a space from one another.

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So you can see again, capital letter

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and one or more lower case letters.

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And that subexpression is
going to be matching up

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with Charlie Cyan in the
search string up above.

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Then we have some literal
characters to match,

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and you can see these
characters match precisely

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this piece of the search string.

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And then we have another subexpression

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in which we are looking for
a primitive email address

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consisting of a one more
more word characters

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an at sign, one or more word characters,

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a period, and three word characters.

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So that would be the com in .com.

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Now notice that we put a
backslash before the dot here,

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that's because the dot is a

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regular expression meta character as well,

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so if we want to use it as a literal,

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which is what we're using here,

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we need to put a backslash in front of it.

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And because we have some backslashes

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in this regular expression,

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we are using a raw string once again.

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So now let's go ahead
and execute a search.

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We're going to locate
that pattern in text.

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Now search is going to
find the first match

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of the entire regular expression,

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so we'll only get a match

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if the entire regular
expression matches the string

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in which we're searching or
a portion of that string.

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If there is a match, only then
will the two subexpressions

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actually get captured, and
once we have that match,

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we can then access those subexpressions.

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So if I say result, and I want to see

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what those subexpression contents were,

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I can use the groups method,
and it will give me back

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all of the matches as
a tuple, in this case.

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Charlie Cyan was the
first subexpression match

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and demo1@deitel.com was the
second subexpression match.

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Now if you want to see
what the entire match was,

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you can say result.group, again, remember

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that the match has to be for
the entire regular expression

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so you can see it matched
the entire string up above.

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And then, finally, if you want to

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individually access the subexpressions

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that were captured, you can do that

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by passing arguments to the group method.

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Now, interestingly, those
arguments are indexed

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from one, not from zero.

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So if I say result.group

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and I give it the argument one,

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that would give me the first match sub,

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first subexpression match,
which was Charlie Cyan,

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and as you'd expect therefore,

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if I give it two the second
piece was demo1@deitel.com.

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So if you're working with structured data

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where each of the strings

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that you're searching
through has the same format

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and you want to extract
information from that data,

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capturing subexpressions is a super easy

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and convenient way to do that.