Regex

Mix.install([
  {:jason, "~> 1.4"},
  {:kino, "~> 0.9", override: true},
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  {:hidden_cell, github: "brooklinjazz/hidden_cell"}
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Review Questions

Upon completing this lesson, a student should be able to answer the following questions.

• What are some common use cases for Regular Expressions?
• How can we use Regular Expressions to manipulate strings?
• How can we experiment and build Regular Expressions using Regexr?

Overview

Regex stands for Regular expressions. Regular expressions are a powerful way to match process strings based on patterns.

We can create a regular expression with the ~r sigil in Elixir.

~r/hello/

We use regular expressions to find matching patterns in a string. We’ll use Regex.scan/3 to show all of the values the regular expression matches on.

For example, the expression ~r/h/ will match the "h" character.

Regex.scan(~r/h/, "hello")

Regex.scan/3 will find all matches in a string.

Below we see that ~r/l/ matches on "l" twice in the "hello" string.

Regex.scan(~r/l/, "hello")

We can match exact characters or words.

Regex.scan(~r/hello/, "hello, world!")

We can also match patterns using special characters. For example, \d will match any digit.

Regex.scan(~r/\d/, "h3ll0")

Building Regular Expressions

Regex differs in implementation depending on the programming language. For example, Elixir uses the PCRE standard (Perl Compatible Regular Expressions).

Regular expressions use special symbols to represent different patterns. Unfortunately, these patterns can become very complex and difficult to read at a glance.

However, regular expressions are also powerful and one of the best ways to process a string.

You can experiment with regular expressions and see a regular expression reference guide on Regexr.

Here are a few patterns to get you started.

• Alphanumeric characters. i.e. h matches "h", - matches "-", and 1 matches "1".
• \d digit.
• \w word character.
• . non-newline character (essentially anything).
• \s whitespace character.

Often capitalizing backslash characters inverts their purpose.

\D non-digit character. W non-word character. \S non-whitespace character.

Regex can find exact matches using non-special characters.

Regex.scan(~r/2/, "123")

\d matches any digit from 0-9.

Regex.scan(~r/\d/, "abcd123")

\w matches any single letter, number, or underscore.

Regex.scan(~r/\w/, "abcd123")

. matches any non-newline\n character.

Regex.scan(~r/./, "abcd\n123")

\s matches whitespaces " ".

Regex.scan(~r/\s/, "a b c")

By combining these symbols, we can create more complex matches. For example, we can match on a phone number in the format 999-999-9999.

Regex.scan(~r/\d\d\d-\d\d\d-\d\d\d\d/, "111-111-1111 222-222-2222 123-123-1234")

Frequency

In addition to matching character patterns, we can match frequency using the following symbols.

• * 0 or more times.
• + one or more times.
• ? zero or one time.
• {} specify the number of times

We can find a match which repeats zero or more times with *.

* uses the previous regular expression, so 1* will match on all strings that contain 1 zero or more times. Similarly, A* would match all strings containing zero or more As.

Regex.scan(~r/1*/, "1121")

+ will match on strings that match one or more of the previous regular expressions.

Regex.scan(~r/1+/, "11211")

? functions like a conditional match, where the match may or may not exist. For example, we could match a and ab with ab?.

Regex.scan(~r/ab?/, "a ab")

{} allows us to specify a specific range of matches. {3} is exactly three matches. {3,} is three or more matches. {3,4} is between three and four matches.

For example, we could use this to create an alternative regular expression for phone numbers.

Regex.scan(~r/\d{3}-\d{3}-\d{4}/, "111-111-1111")

Or match on all digits between 3 and 4 characters.

Regex.scan(~r/\d{3,4}/, "111-111-1111")

Conditions And Ranges

We can also match on ranges of characters or use special conditions.

Use [1-9] to match a range of digits or letters.

Regex.scan(~r/[1-4]/, "1234567")

Regex.scan(~r/[a-c]/, "abcd")

• | match on one pattern or another.

Regex.scan(~r/a|2/, "abc123")

• [^a] match on characters other than the one provided

Regex.scan(~r/[^a]/, "abc123")

Regex Module

The Regex module provides functions for processing strings using regular expressions.

• run/3 Runs the regular expression against the given string until the first match.
• scan/3 Scans the string for all matches.
• replace/4 Replaces all matches.
• split/3 Split the string on matches.
• match?/2 Return a boolean if the string contains the regular expression.

Regex.run/3 will run through a regular expression until the first match.

Regex.run(~r/\d/, "aa1234")

As we’ve already seen, Regex.scan/3 runs through a regular expression finding all matches.

Regex.scan(~r/\d/, "aa1234")

Regex.replace/4 replaces matches with a given string.

Regex.replace(~r/Peter Parker/, "Peter Parker is spiderman", "--secret--")

Regex.replace/3 can also accept an anonymous function to use the matched value to determine how to replace it.

Regex.replace(~r/\d/, "12345", fn each ->
  "#{String.to_integer(each) + 1}"
end)

Regex.split/3 splits a string on all matches the same way that String.split/3 will split a string by a specific value.

String.split("one,two,three", ",")

Regex.split(~r/\d/, "one1two2three")

Regex.match/2 checks if a string contains any match of the regular expression.

Regex.match?(~r/\d/, "hello")

Regex.match?(~r/\d/, "h3ll0")

The String module provides some functions for regular expressions.

• replace/3
• match?/2
• split?/3

These functions are the same as the Regex module, except they take the string as the first argument to make it easier to pipe functions together.

"1234"
|> String.replace(~r/\d/, fn each -> "#{String.to_integer(each) + 1}" end)
|> String.replace(~r/\d/, fn each -> each <> " " end)

Capture Groups

You can build capture groups with (). Capture groups allow you to treat multiple characters as a single unit.

Matching on a capture group may feel unintuitive. For example, using a(b) instead of ab returns multiple matches rather than a single match.

Regex.run(~r/ab/, "ab")

Regex.run(~r/a(b)/, "ab")

Capture groups match separately in regular expressions. So first, we capture ab. Then we capture (b). Notice that if we create another capture group, we also return values that match that group. So (a)(b) matches on ab, then a, then b.

Regex.run(~r/(a)(b)/, "ab")

This could be useful if you wanted to use a specific part of the regular expression when replacing it. For example, let’s obfuscate phone numbers so that 111-111-1111 becomes XXX-111-1111.

We can separate the match into multiple capture groups.

Regex.run(~r/\d{3}-(\d{3}-\d{4})/, "111-111-1111")

Now we can use these capture groups in Regex.replace/3.

Regex.replace(~r/\d{3}-(\d{3}-\d{4})/, "111-111-1111", fn full_match, capture_group ->
  IO.inspect(full_match, label: "match")
  IO.inspect(capture_group, label: "capture group")
  "XXX-" <> capture_group
end)

Regular expressions handle an arbitrary number of capture groups. For example, let’s obfuscate the phone number 111-111-1111 into 111-XXX-1111.

Regex.replace(~r/(\d{3}-)\d{3}(-\d{4})/, "111-111-1111", fn match, group1, group2 ->
  IO.inspect(match, label: "match")
  IO.inspect(group1, label: "group1")
  IO.inspect(group2, label: "group2")
  group1 <> "XXX" <> group2
end)

Your Turn

Use Regex.replace/3 to obfuscate (hide important information) in a phone number. Replace all but the first three digits with X so 999-999-9999 becomes 999-XXX-XXXX.

Example Solution

Regex.replace(~r/(\d{3}-)\d{3}-\d{4}/, "999-999-9999", fn _match, group1 ->
  group1 <> "XXX-" <> "XXXX"
end)

Options

Regex allows for modifiers such as multiline (m) or freespacing x. These modifiers go after the expression i.e ~r/expression/mx.

• m: patterns such as start of line ^ and end of line $ will match multiple lines instead of just the string.
• x: Regular expressions can be written on multiple lines and documented.

Regex.match?(
  ~r/
\+\d{1,3} # country code i.e. +1
\s        # space  
\(\d{3}\) # area code i.e. (555)
-         # dash
(\d{3}    # prefix i.e. 555
-         # dash
\d{4})    # line number i.e. 5555
/x,
  "+1 (111)-111-1111"
)

Lookaround

Lookahead

We can use positive lookahead (?=) to only match expressions followed by another.

For example, we can match all a characters followed by a u character. Notice below that only au will match, and a is the matched value.

Regex.scan(~r/a(?=u)/, "a au")

We can use negative lookahead (?!) to only match on expressions not followed by another.

For example, we can match on a characters not followed by a u character. Notice that at and a match, but not au.

Regex.scan(~r/a(?!u)/, "at a au")

Lookbehind

Positive and negative lookbehind ((?<=) and (?) work similarly to lookahead, except they match expressions preceded by another.

Regex.scan(~r/(?<=u)a/, "ua ta a")

Regex.scan(~r/(?, "ua ta a")

text = "
1. one
2. two
3. three
"

Regex.scan(~r/(?<=\d\. )\w+/, text)

Regex.replace(~r/\#(\d{3})(\d{3})(\d{4})/, "#9999999999", fn _full, group1, group2, group3 -> 
  "#{group1}-#{group2}-#{group3}"
end)

$ git add .
$ git commit -m "finish Regex reading"
$ git push