Difference between revisions of "RegEx Pattern Matching"

Revision as of 10:59, 16 July 2015

#ENCODING
import io
import sys
sys.stdout = io.TextIOWrapper(sys.stdout.buffer, encoding='utf-16')
#MONGO
from pymongo import MongoClient
client = MongoClient()
client.progzoo.authenticate('scott','tiger')
db = client['progzoo']
#PRETTY
import pprint
pp = pprint.PrettyPrinter(indent=4)

Pattern Matching String

This tutorial uses RegEx to check names. We will be using find() on the collection world.

You can use '$regex':"^B" to get all the countries that start with B.

Find the countries that start with Y

pp.pprint(list(
    db.world.find({"name":{'$regex':"^F"}},{"name":1,"_id":0})
))

pp.pprint(list(db.world.find({"name":{'$regex':"^Y"}},{"name":1,"_id":0})))

You can use '$regex':"a$" to get all the countries that end with a.

Find the countries that end with Y

pp.pprint(list(
    db.world.find({"name":{'$regex':"l$"}},{"name":1,"_id":0})
))

pp.pprint(list(db.world.find({"name":{'$regex':"y$"}},{"name":1,"_id":0})))

Luxembourg has an x, so does one other country, list them both

Find the countries that contain the letter x

pp.pprint(list(
    db.world.find({"name":{'$regex':"ana"}},{"name":1,"_id":0})
))

pp.pprint(list(db.world.find({"name":{'$regex':"x"}},{"name":1,"_id":0})))

Iceland and Switzerland end with land but where are the others?

Find the countries that end with land

pp.pprint(list(
    db.world.find({"name":{'$regex':"stan$"}},{"name":1,"_id":0})
))

pp.pprint(list(db.world.find({"name":{'$regex':"land$"}},{"name":1,"_id":0})))

Columbia starts with a C and ends with ia - there are two other countries like this.
You can use .* to match any character except newlines.

Find the countries that start with C and end with ia

pp.pprint(list(
    db.world.find({"name":{'$regex':"^A.*n$"}},{"name":1,"_id":0})
))

pp.pprint(list(db.world.find({"name":{'$regex':"^C.*ia$"}},{"name":1,"_id":0})))

Greece has a double e, who has a double o

Find the countty that has oo in its name

pp.pprint(list(
    db.world.find({"name":{'$regex':"ee"}},{"name":1,"_id":0})
))

pp.pprint(list(db.world.find({"name":{'$regex':"oo"}},{"name":1,"_id":0})))

Bahamas has three a, who else?

Find the country that has three or more a in the name

pp.pprint(list(
    db.world.find({"name":{'$regex':"^T"}},{"name":1,"_id":0})
))

pp.pprint(list(db.world.find({"name":{'$regex':"(.*[aA].*){3}"}},{"name":1,"_id":0})))

India and Angola have n as their second character.
.* Indicates one or more characters, . indicates just one.

Find the countries that have "t" as the second character.

pp.pprint(list(
    db.world.find({"name":{'$regex':"^.n"}},{"name":1,"_id":0})
))

pp.pprint(list(db.world.find({"name":{'$regex':"^.t"}},{"name":1,"_id":0})))

Lesotho and Moldova both have two o characters seperated by two other characters.

Find the countries that have two "o" characters separated by two others.

pp.pprint(list(
    db.world.find({"name":{'$regex':"o.o"}},{"name":1,"_id":0})
))

pp.pprint(list(db.world.find({"name":{'$regex':"o..o"}},{"name":1,"_id":0})))

Cuba and Togo have four character names.

Find the countries that have exactly four characters

pp.pprint(list(
    db.world.find({"name":{'$regex':"^Cu.*$"}},{"name":1,"_id":0})
))

pp.pprint(list(db.world.find({"name":{'$regex':"^.{4}$"}},{"name":1,"_id":0})))

Harder Questions

Well done for getting this far.
Some optional, more complex questions are provided below.

The capital of Luxembourg is Luxembourg. Show all the countries where the capital is the same as the name of the country

You can compare two fields by using where
db.<collection>.find({"$where":"this.<field1> <<operator>> this.<field2>"})
Where uses JavaScript on each document,this means you are able to call string methods such as .match()

Find the country where the name is the capital city.

pp.pprint(list(
    db.world.find({"$where":"this.name == 'Mexico'"},{"name":1,"_id":0})
))

pp.pprint(list(

   db.world.find({"$where":"this.name == this.capital"},{"name":1,"_id":0})

))

The capital of Mexico is Mexico City. Show all the countries where the capital has the country together with the word "City".

Find the country where the capital is the country plus "City"..

pp.pprint(list(
    db.world.find({"$where":"this.capital == 'Mexico'+' City'"},{"name":1,"_id":0})
))

pp.pprint(list(

   db.world.find({"$where":"this.capital == this.name+' City'"},{"name":1,"_id":0})

))

@@ Line 155: / Line 155: @@
 The capital of <b>Luxembourg</b> is <b>Luxembourg</b>. Show all the countries where the capital is the same as the name of the country
 <div class="hint" title="How to compare two fields">
-You can compare two fields by using where <br/><code>db.<collection>.find({"$where":"this.<field1> <<operator>> this.<field2>"})</code>
+You can compare two fields by using where <br/><code>db.<collection>.find({"$where":"this.<field1> <<operator>> this.<field2>"})</code><br/>
+Where uses JavaScript on each document,this means you are able to call string methods such as .match()
 </div>
 <p class=strong>Find the country where the name is the capital city.</p>