class: center, middle, inverse, title-slide

# Strings
### Prof. Maria Tackett

---

class: middle center

## [Click for PDF of slides](22-strings.pdf)

---

class: middle, center

## `stringr`

---

## `stringr`

In addition to the `tidyverse`, we will use the package `stringr`.

```r
library(tidyverse)
library(stringr)
```

`stringr` provides tools to work with character strings.

- Functions in `stringr` have consistent and memorable names

- All begin with `str_` (`str_count`, `str_detect`, `str_trim`, etc)

- All take a vector of strings as their first argument

---

## Preliminaries

Character strings in R are defined by double quotation marks.

They can include letters, numbers, punctuation, whitespace, etc.

```r
string1 <- "STA 199 is my favorite class."
string1
```

```
## [1] "STA 199 is my favorite class."
```

You can combine character strings in a vector.

```r
string2 <- c("STA 199", "Data Science", "Duke")
string2
```

```
## [1] "STA 199"      "Data Science" "Duke"
```

---

## Include a quotation in a string?

Why doesn't the code below work?

.midi[

```r
string3 <- "I said "Hello" to my class"
```
]

--

To include a double quote in a string, *escape it* using a backslash `\`.

--

.midi[

```r
string4 <- "I said \"Hello\" to my class."
```
]

--

What if you want to include an actual backslash?

--

.midi[

```r
string5 <- "\\"
```
]

This may seem tedious but it will come up later!

---

## `writeLines`

`writeLines` shows the contents of the string not
including escapes.

.pull-left[

```r
string4
```

```
## [1] "I said \"Hello\" to my class."
```

```r
writeLines(string4)
```

```
## I said "Hello" to my class.
```
]
.pull-right[

```r
string5
```

```
## [1] "\\"
```

```r
writeLines(string5)
```

```
## \
```
]

---

## U.S. States

To demonstrate functions from `stringr` we will use a vector of all 50 states.

.midi[

```r
states
```

```
##  [1] "alabama"        "alaska"         "arizona"        "arkansas"      
##  [5] "california"     "colorado"       "connecticut"    "delaware"      
##  [9] "florida"        "georgia"        "hawaii"         "idaho"         
## [13] "illinois"       "indiana"        "iowa"           "kansas"        
## [17] "kentucky"       "louisiana"      "maine"          "maryland"      
## [21] "massachusetts"  "michigan"       "minnesota"      "mississippi"   
## [25] "missouri"       "montana"        "nebraska"       "nevada"        
## [29] "new hampshire"  "new jersey"     "new mexico"     "new york"      
## [33] "north carolina" "north dakota"   "ohio"           "oklahoma"      
## [37] "oregon"         "pennsylvania"   "rhode island"   "south carolina"
## [41] "south dakota"   "tennessee"      "texas"          "utah"          
## [45] "vermont"        "virginia"       "washington"     "west virginia" 
## [49] "wisconsin"      "wyoming"
```
]

---

## `str_length`

Given a string, return the number of characters.

.midi[

```r
string1 <- "STA 199 is my favorite class."
str_length(string1)
```

```
## [1] 29
```
]

Given a vector of strings, return the number of characters in each string.

.midi[

```r
str_length(states)
```

```
##  [1]  7  6  7  8 10  8 11  8  7  7  6  5  8  7  4  6  8  9  5  8 13  8  9 11  8
## [26]  7  8  6 13 10 10  8 14 12  4  8  6 12 12 14 12  9  5  4  7  8 10 13  9  7
```
]

--

.pull-left[
- Alabama: 7
- Alaska: 6
- Arizona: 7
- Arkansas: 8
]
.pull-right[
- California: 10
- Colorado: 8
- Connecticut: 11
- ...
]

---

## `str_c`

Combine two or more strings.

```r
str_c("STA 199", "is", "my", "favorite", "class")
```

```
## [1] "STA 199ismyfavoriteclass"
```

--

Use `sep` to specify how the strings are separated.

```r
str_c("STA 199", "is", "my", "favorite", "class", sep = " ")
```

```
## [1] "STA 199 is my favorite class"
```

---

## `str_to_lower` and `str_to_upper`

Convert the case of a string from lower to upper or upper to lower.

.midi[

```r
str_to_upper(states)
```

```
##  [1] "ALABAMA"        "ALASKA"         "ARIZONA"        "ARKANSAS"      
##  [5] "CALIFORNIA"     "COLORADO"       "CONNECTICUT"    "DELAWARE"      
##  [9] "FLORIDA"        "GEORGIA"        "HAWAII"         "IDAHO"         
## [13] "ILLINOIS"       "INDIANA"        "IOWA"           "KANSAS"        
## [17] "KENTUCKY"       "LOUISIANA"      "MAINE"          "MARYLAND"      
## [21] "MASSACHUSETTS"  "MICHIGAN"       "MINNESOTA"      "MISSISSIPPI"   
## [25] "MISSOURI"       "MONTANA"        "NEBRASKA"       "NEVADA"        
## [29] "NEW HAMPSHIRE"  "NEW JERSEY"     "NEW MEXICO"     "NEW YORK"      
## [33] "NORTH CAROLINA" "NORTH DAKOTA"   "OHIO"           "OKLAHOMA"      
## [37] "OREGON"         "PENNSYLVANIA"   "RHODE ISLAND"   "SOUTH CAROLINA"
## [41] "SOUTH DAKOTA"   "TENNESSEE"      "TEXAS"          "UTAH"          
## [45] "VERMONT"        "VIRGINIA"       "WASHINGTON"     "WEST VIRGINIA" 
## [49] "WISCONSIN"      "WYOMING"
```
]

---

## `str_sub`

Extract parts of a string from `start` to `end`, inclusive.

.midi[

```r
str_sub(states, 1, 4)
```

```
##  [1] "alab" "alas" "ariz" "arka" "cali" "colo" "conn" "dela" "flor" "geor"
## [11] "hawa" "idah" "illi" "indi" "iowa" "kans" "kent" "loui" "main" "mary"
## [21] "mass" "mich" "minn" "miss" "miss" "mont" "nebr" "neva" "new " "new "
## [31] "new " "new " "nort" "nort" "ohio" "okla" "oreg" "penn" "rhod" "sout"
## [41] "sout" "tenn" "texa" "utah" "verm" "virg" "wash" "west" "wisc" "wyom"
```
]

--

.midi[

```r
str_sub(states, -4, -1)
```

```
##  [1] "bama" "aska" "zona" "nsas" "rnia" "rado" "icut" "ware" "rida" "rgia"
## [11] "waii" "daho" "nois" "iana" "iowa" "nsas" "ucky" "iana" "aine" "land"
## [21] "etts" "igan" "sota" "ippi" "ouri" "tana" "aska" "vada" "hire" "rsey"
## [31] "xico" "york" "lina" "kota" "ohio" "homa" "egon" "ania" "land" "lina"
## [41] "kota" "ssee" "exas" "utah" "mont" "inia" "gton" "inia" "nsin" "ming"
```
]

---

## `str_sub` and `str_to_upper`

Can combine `str_sub` and `str_to_upper` to capitalize each state.

.midi[

```r
str_sub(states, 1, 1) <- str_to_upper(str_sub(states, 1, 1))
states
```

```
##  [1] "Alabama"        "Alaska"         "Arizona"        "Arkansas"      
##  [5] "California"     "Colorado"       "Connecticut"    "Delaware"      
##  [9] "Florida"        "Georgia"        "Hawaii"         "Idaho"         
## [13] "Illinois"       "Indiana"        "Iowa"           "Kansas"        
## [17] "Kentucky"       "Louisiana"      "Maine"          "Maryland"      
## [21] "Massachusetts"  "Michigan"       "Minnesota"      "Mississippi"   
## [25] "Missouri"       "Montana"        "Nebraska"       "Nevada"        
## [29] "New hampshire"  "New jersey"     "New mexico"     "New york"      
## [33] "North carolina" "North dakota"   "Ohio"           "Oklahoma"      
## [37] "Oregon"         "Pennsylvania"   "Rhode island"   "South carolina"
## [41] "South dakota"   "Tennessee"      "Texas"          "Utah"          
## [45] "Vermont"        "Virginia"       "Washington"     "West virginia" 
## [49] "Wisconsin"      "Wyoming"
```
]

---

## `str_sort`

Sort a string. Here we sort in decreasing alphabetical order.

.midi[

```r
str_sort(states, decreasing = TRUE)
```

```
##  [1] "Wyoming"        "Wisconsin"      "West virginia"  "Washington"    
##  [5] "Virginia"       "Vermont"        "Utah"           "Texas"         
##  [9] "Tennessee"      "South dakota"   "South carolina" "Rhode island"  
## [13] "Pennsylvania"   "Oregon"         "Oklahoma"       "Ohio"          
## [17] "North dakota"   "North carolina" "New york"       "New mexico"    
## [21] "New jersey"     "New hampshire"  "Nevada"         "Nebraska"      
## [25] "Montana"        "Missouri"       "Mississippi"    "Minnesota"     
## [29] "Michigan"       "Massachusetts"  "Maryland"       "Maine"         
## [33] "Louisiana"      "Kentucky"       "Kansas"         "Iowa"          
## [37] "Indiana"        "Illinois"       "Idaho"          "Hawaii"        
## [41] "Georgia"        "Florida"        "Delaware"       "Connecticut"   
## [45] "Colorado"       "California"     "Arkansas"       "Arizona"       
## [49] "Alaska"         "Alabama"
```
]

---

## Regular Expressions

A .vocab[regular expression] is a sequence of characters that allows you to 
describe string patterns. We use them to search for patterns.

- extract a phone number from text data
- determine if an email address is valid
- determine if a password has the required number of letters, characters, and symbols
- count the number of times "statistics" occurs in a corpus of text
- ...

---

## Regular Expressions

To demonstrate will will use a vector of all of the states bordering North 
Carolina.

```r
nc_states <- c("North Carolina", "South Carolina", 
               "Virginia", "Tennessee", "Georgia")
nc_states
```

```
## [1] "North Carolina" "South Carolina" "Virginia"       "Tennessee"     
## [5] "Georgia"
```

---

## Basic Match

We can match exactly.

```r
str_view_all(nc_states, "in")
```

<div id="htmlwidget-e313d0a5c26bda0a51f1" style="width:960px;height:100%;" class="str_view html-widget"></div>
<script type="application/json" data-for="htmlwidget-e313d0a5c26bda0a51f1">{"x":{"html":"<ul>\n  <li>North Carol<span class='match'>in<\/span>a<\/li>\n  <li>South Carol<span class='match'>in<\/span>a<\/li>\n  <li>Virg<span class='match'>in<\/span>ia<\/li>\n  <li>Tennessee<\/li>\n  <li>Georgia<\/li>\n<\/ul>"},"evals":[],"jsHooks":[]}</script>

---

## Basic Match

Match any character using `.`

```r
str_view_all(nc_states, "i.")
```

<div id="htmlwidget-55edff1d49f114b90c01" style="width:960px;height:100%;" class="str_view html-widget"></div>
<script type="application/json" data-for="htmlwidget-55edff1d49f114b90c01">{"x":{"html":"<ul>\n  <li>North Carol<span class='match'>in<\/span>a<\/li>\n  <li>South Carol<span class='match'>in<\/span>a<\/li>\n  <li>V<span class='match'>ir<\/span>g<span class='match'>in<\/span><span class='match'>ia<\/span><\/li>\n  <li>Tennessee<\/li>\n  <li>Georg<span class='match'>ia<\/span><\/li>\n<\/ul>"},"evals":[],"jsHooks":[]}</script>

---

## Anchors

Match the start of a string using `^`

```r
str_view_all(nc_states, "^G")
```

<div id="htmlwidget-71a7086a3e551139430f" style="width:960px;height:100%;" class="str_view html-widget"></div>
<script type="application/json" data-for="htmlwidget-71a7086a3e551139430f">{"x":{"html":"<ul>\n  <li>North Carolina<\/li>\n  <li>South Carolina<\/li>\n  <li>Virginia<\/li>\n  <li>Tennessee<\/li>\n  <li><span class='match'>G<\/span>eorgia<\/li>\n<\/ul>"},"evals":[],"jsHooks":[]}</script>

---

## Anchors

Match the end of a string using `$`

```r
str_view_all(nc_states, "a$")
```

<div id="htmlwidget-8b72e137f043766338db" style="width:960px;height:100%;" class="str_view html-widget"></div>
<script type="application/json" data-for="htmlwidget-8b72e137f043766338db">{"x":{"html":"<ul>\n  <li>North Carolin<span class='match'>a<\/span><\/li>\n  <li>South Carolin<span class='match'>a<\/span><\/li>\n  <li>Virgini<span class='match'>a<\/span><\/li>\n  <li>Tennessee<\/li>\n  <li>Georgi<span class='match'>a<\/span><\/li>\n<\/ul>"},"evals":[],"jsHooks":[]}</script>

---

## `str_detect`

Determine if a character vector matches a pattern.

```r
nc_states
```

```
## [1] "North Carolina" "South Carolina" "Virginia"       "Tennessee"     
## [5] "Georgia"
```

```r
str_detect(nc_states, "a")
```

```
## [1]  TRUE  TRUE  TRUE FALSE  TRUE
```

---

## `str_subset`

Select elements that match a pattern.

```r
str_subset(nc_states, "e$")
```

```
## [1] "Tennessee"
```

---

## `str_count`

How many matches are there in a string?

```r
nc_states
```

```
## [1] "North Carolina" "South Carolina" "Virginia"       "Tennessee"     
## [5] "Georgia"
```

```r
str_count(nc_states, "a")
```

```
## [1] 2 2 1 0 1
```

---

## `str_replace`

Replace first match with new strings.

```r
str_replace(nc_states, "a", "-")
```

```
## [1] "North C-rolina" "South C-rolina" "Virgini-"       "Tennessee"     
## [5] "Georgi-"
```

---

## `str_replace_all`

Replace all matches with new strings.

```r
str_replace_all(nc_states, "a", "-")
```

```
## [1] "North C-rolin-" "South C-rolin-" "Virgini-"       "Tennessee"     
## [5] "Georgi-"
```

---

## Many Matches

The regular expressions below match more than one character.

- Match any digit using `\d` or `[[:digit:]]`
- Match any whitespace using `\s` or `[[:space:]]`
- Match f, g, or h using `[fgh]` 
- Match anything but f, g, or h using `[^fgh]`
- Match lower-case letters using `[a-z]` or `[[:lower:]]`
- Match upper-case letters using `[A-Z]` or `[[:upper:]]`
- Match alphabetic characters using `[A-z]` or `[[:alpha:]]`

Remember these are regular expressions! To match digits you'll need to *escape*
the string, so use `"\\d"`, not `"\d"`

---

## Additional resources

- `stringr` website: https://stringr.tidyverse.org/
- `stringr` [cheat sheet](https://raw.githubusercontent.com/rstudio/cheatsheets/master/strings.pdf)
- Regular Expressions [Cheat Sheet](https://rstudio.com/wp-content/uploads/2016/09/RegExCheatsheet.pdf)
- [Chapter 14: Strings](https://r4ds.had.co.nz/strings.html#matching-patterns-with-regular-expressions) in R for Data Science

Notes for current slide

Notes for next slide

StringsProf. Maria Tackett1

Click for PDF of slides

2

stringr3

`stringr`

In addition to the tidyverse, we will use the package stringr.

library(tidyverse)
library(stringr)

stringr provides tools to work with character strings.

Functions in stringr have consistent and memorable names
All begin with str_ (str_count, str_detect, str_trim, etc)
All take a vector of strings as their first argument

4

Preliminaries

Character strings in R are defined by double quotation marks.

They can include letters, numbers, punctuation, whitespace, etc.

string1 <- "STA 199 is my favorite class."
string1

## [1] "STA 199 is my favorite class."

You can combine character strings in a vector.

string2 <- c("STA 199", "Data Science", "Duke")
string2

## [1] "STA 199"      "Data Science" "Duke"

5

Include a quotation in a string?

Why doesn't the code below work?

string3 <- "I said "Hello" to my class"

6

Include a quotation in a string?

Why doesn't the code below work?

string3 <- "I said "Hello" to my class"

To include a double quote in a string, escape it using a backslash \.

6

Include a quotation in a string?

Why doesn't the code below work?

string3 <- "I said "Hello" to my class"

To include a double quote in a string, escape it using a backslash \.

string4 <- "I said \"Hello\" to my class."

6

Include a quotation in a string?

Why doesn't the code below work?

string3 <- "I said "Hello" to my class"

To include a double quote in a string, escape it using a backslash \.

string4 <- "I said \"Hello\" to my class."

What if you want to include an actual backslash?

6

Include a quotation in a string?

Why doesn't the code below work?

string3 <- "I said "Hello" to my class"

To include a double quote in a string, escape it using a backslash \.

string4 <- "I said \"Hello\" to my class."

What if you want to include an actual backslash?

string5 <- "\\"

This may seem tedious but it will come up later!

6

`writeLines`

writeLines shows the contents of the string not including escapes.

string4

## [1] "I said \"Hello\" to my class."

writeLines(string4)

## I said "Hello" to my class.

string5

## [1] "\\"

writeLines(string5)

## \

7

U.S. States

To demonstrate functions from stringr we will use a vector of all 50 states.

states

##  [1] "alabama"        "alaska"         "arizona"        "arkansas"      
##  [5] "california"     "colorado"       "connecticut"    "delaware"      
##  [9] "florida"        "georgia"        "hawaii"         "idaho"         
## [13] "illinois"       "indiana"        "iowa"           "kansas"        
## [17] "kentucky"       "louisiana"      "maine"          "maryland"      
## [21] "massachusetts"  "michigan"       "minnesota"      "mississippi"   
## [25] "missouri"       "montana"        "nebraska"       "nevada"        
## [29] "new hampshire"  "new jersey"     "new mexico"     "new york"      
## [33] "north carolina" "north dakota"   "ohio"           "oklahoma"      
## [37] "oregon"         "pennsylvania"   "rhode island"   "south carolina"
## [41] "south dakota"   "tennessee"      "texas"          "utah"          
## [45] "vermont"        "virginia"       "washington"     "west virginia" 
## [49] "wisconsin"      "wyoming"

8

`str_length`

Given a string, return the number of characters.

string1 <- "STA 199 is my favorite class."
str_length(string1)

## [1] 29

Given a vector of strings, return the number of characters in each string.

str_length(states)

##  [1]  7  6  7  8 10  8 11  8  7  7  6  5  8  7  4  6  8  9  5  8 13  8  9 11  8
## [26]  7  8  6 13 10 10  8 14 12  4  8  6 12 12 14 12  9  5  4  7  8 10 13  9  7

9

`str_length`

Given a string, return the number of characters.

string1 <- "STA 199 is my favorite class."
str_length(string1)

## [1] 29

Given a vector of strings, return the number of characters in each string.

str_length(states)

##  [1]  7  6  7  8 10  8 11  8  7  7  6  5  8  7  4  6  8  9  5  8 13  8  9 11  8
## [26]  7  8  6 13 10 10  8 14 12  4  8  6 12 12 14 12  9  5  4  7  8 10 13  9  7

Alabama: 7
Alaska: 6
Arizona: 7
Arkansas: 8

California: 10
Colorado: 8
Connecticut: 11
...

9

`str_c`

Combine two or more strings.

str_c("STA 199", "is", "my", "favorite", "class")

## [1] "STA 199ismyfavoriteclass"

10

`str_c`

Combine two or more strings.

str_c("STA 199", "is", "my", "favorite", "class")

## [1] "STA 199ismyfavoriteclass"

Use sep to specify how the strings are separated.

str_c("STA 199", "is", "my", "favorite", "class", sep = " ")

## [1] "STA 199 is my favorite class"

10

`str_to_lower` and `str_to_upper`

Convert the case of a string from lower to upper or upper to lower.

str_to_upper(states)

##  [1] "ALABAMA"        "ALASKA"         "ARIZONA"        "ARKANSAS"      
##  [5] "CALIFORNIA"     "COLORADO"       "CONNECTICUT"    "DELAWARE"      
##  [9] "FLORIDA"        "GEORGIA"        "HAWAII"         "IDAHO"         
## [13] "ILLINOIS"       "INDIANA"        "IOWA"           "KANSAS"        
## [17] "KENTUCKY"       "LOUISIANA"      "MAINE"          "MARYLAND"      
## [21] "MASSACHUSETTS"  "MICHIGAN"       "MINNESOTA"      "MISSISSIPPI"   
## [25] "MISSOURI"       "MONTANA"        "NEBRASKA"       "NEVADA"        
## [29] "NEW HAMPSHIRE"  "NEW JERSEY"     "NEW MEXICO"     "NEW YORK"      
## [33] "NORTH CAROLINA" "NORTH DAKOTA"   "OHIO"           "OKLAHOMA"      
## [37] "OREGON"         "PENNSYLVANIA"   "RHODE ISLAND"   "SOUTH CAROLINA"
## [41] "SOUTH DAKOTA"   "TENNESSEE"      "TEXAS"          "UTAH"          
## [45] "VERMONT"        "VIRGINIA"       "WASHINGTON"     "WEST VIRGINIA" 
## [49] "WISCONSIN"      "WYOMING"

11

`str_sub`

Extract parts of a string from start to end, inclusive.

str_sub(states, 1, 4)

##  [1] "alab" "alas" "ariz" "arka" "cali" "colo" "conn" "dela" "flor" "geor"
## [11] "hawa" "idah" "illi" "indi" "iowa" "kans" "kent" "loui" "main" "mary"
## [21] "mass" "mich" "minn" "miss" "miss" "mont" "nebr" "neva" "new " "new "
## [31] "new " "new " "nort" "nort" "ohio" "okla" "oreg" "penn" "rhod" "sout"
## [41] "sout" "tenn" "texa" "utah" "verm" "virg" "wash" "west" "wisc" "wyom"

12

`str_sub`

Extract parts of a string from start to end, inclusive.

str_sub(states, 1, 4)

##  [1] "alab" "alas" "ariz" "arka" "cali" "colo" "conn" "dela" "flor" "geor"
## [11] "hawa" "idah" "illi" "indi" "iowa" "kans" "kent" "loui" "main" "mary"
## [21] "mass" "mich" "minn" "miss" "miss" "mont" "nebr" "neva" "new " "new "
## [31] "new " "new " "nort" "nort" "ohio" "okla" "oreg" "penn" "rhod" "sout"
## [41] "sout" "tenn" "texa" "utah" "verm" "virg" "wash" "west" "wisc" "wyom"

str_sub(states, -4, -1)

##  [1] "bama" "aska" "zona" "nsas" "rnia" "rado" "icut" "ware" "rida" "rgia"
## [11] "waii" "daho" "nois" "iana" "iowa" "nsas" "ucky" "iana" "aine" "land"
## [21] "etts" "igan" "sota" "ippi" "ouri" "tana" "aska" "vada" "hire" "rsey"
## [31] "xico" "york" "lina" "kota" "ohio" "homa" "egon" "ania" "land" "lina"
## [41] "kota" "ssee" "exas" "utah" "mont" "inia" "gton" "inia" "nsin" "ming"

12

`str_sub` and `str_to_upper`

Can combine str_sub and str_to_upper to capitalize each state.

str_sub(states, 1, 1) <- str_to_upper(str_sub(states, 1, 1))
states

##  [1] "Alabama"        "Alaska"         "Arizona"        "Arkansas"      
##  [5] "California"     "Colorado"       "Connecticut"    "Delaware"      
##  [9] "Florida"        "Georgia"        "Hawaii"         "Idaho"         
## [13] "Illinois"       "Indiana"        "Iowa"           "Kansas"        
## [17] "Kentucky"       "Louisiana"      "Maine"          "Maryland"      
## [21] "Massachusetts"  "Michigan"       "Minnesota"      "Mississippi"   
## [25] "Missouri"       "Montana"        "Nebraska"       "Nevada"        
## [29] "New hampshire"  "New jersey"     "New mexico"     "New york"      
## [33] "North carolina" "North dakota"   "Ohio"           "Oklahoma"      
## [37] "Oregon"         "Pennsylvania"   "Rhode island"   "South carolina"
## [41] "South dakota"   "Tennessee"      "Texas"          "Utah"          
## [45] "Vermont"        "Virginia"       "Washington"     "West virginia" 
## [49] "Wisconsin"      "Wyoming"

13

`str_sort`

Sort a string. Here we sort in decreasing alphabetical order.

str_sort(states, decreasing = TRUE)

##  [1] "Wyoming"        "Wisconsin"      "West virginia"  "Washington"    
##  [5] "Virginia"       "Vermont"        "Utah"           "Texas"         
##  [9] "Tennessee"      "South dakota"   "South carolina" "Rhode island"  
## [13] "Pennsylvania"   "Oregon"         "Oklahoma"       "Ohio"          
## [17] "North dakota"   "North carolina" "New york"       "New mexico"    
## [21] "New jersey"     "New hampshire"  "Nevada"         "Nebraska"      
## [25] "Montana"        "Missouri"       "Mississippi"    "Minnesota"     
## [29] "Michigan"       "Massachusetts"  "Maryland"       "Maine"         
## [33] "Louisiana"      "Kentucky"       "Kansas"         "Iowa"          
## [37] "Indiana"        "Illinois"       "Idaho"          "Hawaii"        
## [41] "Georgia"        "Florida"        "Delaware"       "Connecticut"   
## [45] "Colorado"       "California"     "Arkansas"       "Arizona"       
## [49] "Alaska"         "Alabama"

14

Regular Expressions

A regular expression is a sequence of characters that allows you to describe string patterns. We use them to search for patterns.

extract a phone number from text data
determine if an email address is valid
determine if a password has the required number of letters, characters, and symbols
count the number of times "statistics" occurs in a corpus of text
...

15

Regular Expressions

To demonstrate will will use a vector of all of the states bordering North Carolina.

nc_states <- c("North Carolina", "South Carolina", 
               "Virginia", "Tennessee", "Georgia")
nc_states

## [1] "North Carolina" "South Carolina" "Virginia"       "Tennessee"     
## [5] "Georgia"

16

Basic Match

We can match exactly.

str_view_all(nc_states, "in")

17

Basic Match

Match any character using .

str_view_all(nc_states, "i.")

18

Anchors

Match the start of a string using ^

str_view_all(nc_states, "^G")

19

Anchors

Match the end of a string using $

str_view_all(nc_states, "a$")

20

`str_detect`

Determine if a character vector matches a pattern.

nc_states

## [1] "North Carolina" "South Carolina" "Virginia"       "Tennessee"     
## [5] "Georgia"

str_detect(nc_states, "a")

## [1]  TRUE  TRUE  TRUE FALSE  TRUE

21

`str_subset`

Select elements that match a pattern.

str_subset(nc_states, "e$")

## [1] "Tennessee"

22

`str_count`

How many matches are there in a string?

nc_states

## [1] "North Carolina" "South Carolina" "Virginia"       "Tennessee"     
## [5] "Georgia"

str_count(nc_states, "a")

## [1] 2 2 1 0 1

23

`str_replace`

Replace first match with new strings.

str_replace(nc_states, "a", "-")

## [1] "North C-rolina" "South C-rolina" "Virgini-"       "Tennessee"     
## [5] "Georgi-"

24

`str_replace_all`

Replace all matches with new strings.

str_replace_all(nc_states, "a", "-")

## [1] "North C-rolin-" "South C-rolin-" "Virgini-"       "Tennessee"     
## [5] "Georgi-"

25

Many Matches

The regular expressions below match more than one character.

Match any digit using \d or [[:digit:]]
Match any whitespace using \s or [[:space:]]
Match f, g, or h using [fgh]
Match anything but f, g, or h using [^fgh]
Match lower-case letters using [a-z] or [[:lower:]]
Match upper-case letters using [A-Z] or [[:upper:]]
Match alphabetic characters using [A-z] or [[:alpha:]]

Remember these are regular expressions! To match digits you'll need to escape the string, so use "\\d", not "\d"

26

Additional resources

stringr website: https://stringr.tidyverse.org/
stringr cheat sheet
Regular Expressions Cheat Sheet
Chapter 14: Strings in R for Data Science

27

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