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<!DOCTYPE html>
<html lang="" xml:lang="">
<head>
<title>String manipulation and regexes</title>
<meta charset="utf-8" />
<meta name="author" content="Shawn Santo" />
<script src="libs/htmlwidgets-1.5.1/htmlwidgets.js"></script>
<link href="libs/str_view-0.1.0/str_view.css" rel="stylesheet" />
<script src="libs/str_view-binding-1.4.0/str_view.js"></script>
<link rel="stylesheet" href="slides.css" type="text/css" />
</head>
<body>
<textarea id="source">
class: center, middle, inverse, title-slide
# String manipulation and regexes
## Programming for Statistical Science
### Shawn Santo
---
## Supplementary materials
Full video lecture available in Zoom Cloud Recordings
Additional resources
- `stringr` [vignette](https://cran.r-project.org/web/packages/stringr/vignettes/stringr.html)
- `stringr` [cheat sheet](https://github.com/rstudio/cheatsheets/raw/master/strings.pdf)
- regex [guide](http://perso.ens-lyon.fr/lise.vaudor/Rfigures/Expressions_regulieres/regexp.png)
---
class: inverse, center, middle
# `stringr`
---
## Why `stringr`?
- Part of `tidyverse`
- Fast and consistent manipulation of string data
- Readable and consistent syntax
- If you master `stringr`, you know `stringi` - http://www.gagolewski.com/software/stringi/
---
## Usage
- All functions in `stringr` start with `str_` and take a vector of strings
as the first argument.
- Most `stringr` functions work with regular expressions.
- Seven main verbs to work with strings.
.small-text[
| Function | Description |
|----------------:|:-------------------------------------------------------------------------------|
| `str_detect()` | Detect the presence or absence of a pattern in a string. |
| `str_count()` | Count the number of patterns. |
| `str_locate()` | Locate the first position of a pattern and return a matrix with start and end. |
| `str_extract()` | Extracts text corresponding to the first match. |
| `str_match()` | Extracts capture groups formed by `()` from the first match. |
| `str_split()` | Splits string into pieces and returns a list of character vectors. |
| `str_replace()` | Replaces the first matched pattern and returns a character vector. |
]
<br/>
Each have leading arguments `string` and `pattern`; all functions are vectorised
over arguments `string` and `pattern`.
---
class: inverse, center, middle
# Regexs
---
## Simple cases
A regular expression, regex or regexp, is a sequence of characters that
define a search pattern.
```r
library(tidyverse)
```
.small[
```r
twister <- "thirty-three thieves thought they thrilled the throne Thursday"
```
]
--
How many occurrences of `t` exist?
.small[
```r
str_count(string = twister, pattern = "t")
```
```
#> [1] 10
```
]
--
.pull-left[
How many of `t`, `th`, and `the` exist?
.small[
```r
str_count(twister, c("t", "th", "the"))
```
```
#> [1] 10 8 2
```
]
]
.pull-right[
Do these patterns exist?
.small[
```r
str_detect(twister, c("t", "th", "the"))
```
```
#> [1] TRUE TRUE TRUE
```
]
]
---
Separate our long string at each space.
```r
twister_split <- str_split(twister, " ") %>% unlist()
twister_split
```
```
#> [1] "thirty-three" "thieves" "thought" "they" "thrilled"
#> [6] "the" "throne" "Thursday"
```
--
<br/>
Do these patterns exist?
```r
str_detect(twister_split, c("tho", "the"))
```
```
#> [1] FALSE FALSE TRUE TRUE FALSE TRUE FALSE FALSE
```
--
<br/>
Replace certain occurrences.
.small[
```r
str_replace(twister_split, c("tho", "the"), replacement = c("bro", "Wil"))
```
```
#> [1] "thirty-three" "thieves" "brought" "Wily" "thrilled"
#> [6] "Wil" "throne" "Thursday"
```
]
---
## A step up in complexity
A `.` matches any character, except a new line. It is one of a few
metacharacters - special meaning and function.
.small[
```r
twister <- "thirty-three thieves thought they thrilled the throne Thursday"
```
]
Does this pattern, `.y.` exist?
```r
str_detect(twister, ".y.")
```
```
#> [1] TRUE
```
--
How many instances?
```r
str_count(twister, ".y.")
```
```
#> [1] 2
```
--
View in Viewer pane.
```r
str_view_all(twister, ".y.")
```
<div id="htmlwidget-67bd2798ace63c1417e2" style="width:960px;height:100%;" class="str_view html-widget"></div>
<script type="application/json" data-for="htmlwidget-67bd2798ace63c1417e2">{"x":{"html":"<ul>\n <li>thir<span class='match'>ty-<\/span>three thieves thought th<span class='match'>ey <\/span>thrilled the throne Thursday<\/li>\n<\/ul>"},"evals":[],"jsHooks":[]}</script>
---
## How do we match an actual `.`?
You need to use an escape character to tell the regex you want exact matching.
Regexs use a `\` as an escape character. So why doesn't this work?
```r
str_view_all("show.me.the.dots...", "\.")
```
```
#> Error: '\.' is an unrecognized escape in character string starting ""\."
```
---
## R escape characters
**There are some special characters in R that cannot be directly coded in a
string**. An escape character is a character which results in an alternative
interpretation of the following character(s). These vary from language to
language, but **for most string implementations `\` is the escape character
which is modified by a single subsequent character.**
Some common examples:
.small[
| Literal | Character |
|:--------|:-----------------|
|`\'` | single quote |
|`\"` | double quote |
|`\\` | backslash |
|`\n` | new line |
|`\r` | carriage return |
|`\t` | tab |
|`\b` | backspace |
|`\f` | form feed |
]
???
Escaping a character not in the following table is an error.
| Literal | Character |
|:--------|:-----------------|
|`\n` | newline |
|`\r` | carriage return |
|`\t` | tab |
|`\b` | backspace |
|`\a` | alert (bell) |
|`\f` | form feed |
|`\v` | vertical tab |
|`\\` | backslash \ |
|`\'` | ASCII apostrophe ' |
|`\"` | ASCII quotation mark " |
|`\\\\`` | ASCII grave accent (backtick) \` |
|`\nnn` | character with given octal code (1, 2 or 3 digits) |
|`\xnn` | character with given hex code (1 or 2 hex digits) |
|`\unnnn` | Unicode character with given code (1--4 hex digits) |
|`\Unnnnnnnn` | Unicode character with given code (1--8 hex digits) |
---
## Examples
.tiny[
```r
mtcars %>%
ggplot(aes(x = factor(cyl), y = hp)) + ggpol::geom_boxjitter() +
* labs(x = "Number \n of \n Cylinders", y = "\"Horse\" Power",
* title = "A \t boxjitter \t\t plot \n showing some escape \n characters") +
theme_minimal(base_size = 18)
```
<img src="lec_10_files/figure-html/unnamed-chunk-15-1.png" style="display: block; margin: auto;" />
]
---
## Examples
```r
print("hello\world")
```
```
#> Error: '\w' is an unrecognized escape in character string starting ""hello\w"
```
```r
cat("hello\world")
```
```
#> Error: '\w' is an unrecognized escape in character string starting ""hello\w"
```
--
```r
print("hello\tworld")
```
```
#> [1] "hello\tworld"
```
```r
cat("hello\tworld")
```
```
#> hello world
```
---
.pull-left[
A quote
```r
print("hello\"world")
```
```
#> [1] "hello\"world"
```
```r
cat("hello\"world")
```
```
#> hello"world
```
<br/>
A new line
```r
print("hello\nworld")
```
```
#> [1] "hello\nworld"
```
```r
cat("hello\nworld")
```
```
#> hello
#> world
```
]
.pull-right[
A backslash
```r
print("hello\\world")
```
```
#> [1] "hello\\world"
```
```r
cat("hello\\world")
```
```
#> hello\world
```
]
---
## Returning to: how do we match a `.`?
We need to escape the backslash in our regex of `\`.
```r
str_view_all("show.me.the.dots...", "\\.")
```
<div id="htmlwidget-bd128268a2115e60fc1f" style="width:960px;height:100%;" class="str_view html-widget"></div>
<script type="application/json" data-for="htmlwidget-bd128268a2115e60fc1f">{"x":{"html":"<ul>\n <li>show<span class='match'>.<\/span>me<span class='match'>.<\/span>the<span class='match'>.<\/span>dots<span class='match'>.<\/span><span class='match'>.<\/span><span class='match'>.<\/span><\/li>\n<\/ul>"},"evals":[],"jsHooks":[]}</script>
---
## Regex metacharacters
```markdown
. ^ $ * + ? { } [ ] \ | ( )
```
Allow for more advanced forms of pattern matching.
<br/>
As we saw with `.`, these cannot be matched directly. Thus, if you want to match
the literal `?` you will need to use `\\?`.
--
<br/><br/>
What do you need to match a literal `\` in regex pattern matching?
--
```r
str_view_all("find the \\ in this string", "\\\\")
```
<div id="htmlwidget-1667a773014b54df86b9" style="width:960px;height:100%;" class="str_view html-widget"></div>
<script type="application/json" data-for="htmlwidget-1667a773014b54df86b9">{"x":{"html":"<ul>\n <li>find the <span class='match'>\\<\/span> in this string<\/li>\n<\/ul>"},"evals":[],"jsHooks":[]}</script>
---
## Regex anchors
Sometimes we want to specify that our pattern occurs at a particular
location in a string, we indicate this using anchor metacharacters.
<br />
| Regex | Anchor |
|-------------|-----------------|
| `^` or `\A` | Start of string |
| `$` or `\Z` | End of string |
---
## Example: anchors
```r
text <- c("Which?", "Witch", "Will", "SWitch?")
```
```r
str_replace(text, "W...", "****")
```
```
#> [1] "****h?" "****h" "****" "S****h?"
```
```r
str_replace(text, "^W...", "****")
```
```
#> [1] "****h?" "****h" "****" "SWitch?"
```
--
<br/>
```r
str_replace(text, "W...h", "****")
```
```
#> [1] "****?" "****" "Will" "S****?"
```
```r
str_replace(text, "W...h$", "****")
```
```
#> [1] "Which?" "****" "Will" "SWitch?"
```
---
## Character classes
Special patterns exist to match more than one class.
<br />
| Meta Character | Class | Description |
|:--------------:|-------------|--------------------------------------|
| `.` | | Any character except new line (`\n`) |
| `\s` | `[:space:]` | White space (space, tab, newline) |
| `\S` | | Not white space |
| `\d` | `[:digit:]` | Digit (0-9) |
| `\D` | | Not digit |
| `\w` | | Word (A-Z, a-z, 0-9, or _) |
| `\W` | | Not word |
---
## Character class overview
<center>
<img src="images/regex_char_classes.png" width=450>
</center>
---
## Ranges
We can also specify our own classes using the square bracket metacharacter.
<br />
| Class | Type |
|----------|-------------------------------------|
| `[abc]` | Class (a or b or c) |
| `[^abc]` | Negated class not (a or b or c) |
| `[a-c]` | Range lower case letter from a to c |
| `[A-C]` | Range upper case letter from A to C |
| `[0-7]` | Digit between 0 to 7 |
---
## Exercises
Write a regular expression to match a
1. social security number of the form ###-##-####,
2. phone number of the form (###) ###-####,
3. license plate of the form AAA ####.
Test your regexs on some examples with `str_detect()` or `str_view()`.
???
## Solutions
```r
x <- "My info is as follows. Cell: (432)-431-1512. Social security: 432-11-1990"
y <- "Vehicle info: AEF 2348"
# not the most efficient
str_view_all(x, "[0-9][0-9][0-9]-[0-9][0-9]-[0-9][0-9][0-9][0-9]")
```
<div id="htmlwidget-83168efebffcacc9521c" style="width:960px;height:100%;" class="str_view html-widget"></div>
<script type="application/json" data-for="htmlwidget-83168efebffcacc9521c">{"x":{"html":"<ul>\n <li>My info is as follows. Cell: (432)-431-1512. Social security: <span class='match'>432-11-1990<\/span><\/li>\n<\/ul>"},"evals":[],"jsHooks":[]}</script>
```r
str_view_all(x, "\\([0-9][0-9][0-9]\\)-[0-9][0-9][0-9]-[0-9][0-9][0-9][0-9]")
```
<div id="htmlwidget-da2f7f06b9597690941d" style="width:960px;height:100%;" class="str_view html-widget"></div>
<script type="application/json" data-for="htmlwidget-da2f7f06b9597690941d">{"x":{"html":"<ul>\n <li>My info is as follows. Cell: <span class='match'>(432)-431-1512<\/span>. Social security: 432-11-1990<\/li>\n<\/ul>"},"evals":[],"jsHooks":[]}</script>
```r
str_view_all(y, "[A-Z][A-Z][A-Z] [0-9][0-9][0-9][0-9]")
```
<div id="htmlwidget-f2e0aad48a15f9f2d949" style="width:960px;height:100%;" class="str_view html-widget"></div>
<script type="application/json" data-for="htmlwidget-f2e0aad48a15f9f2d949">{"x":{"html":"<ul>\n <li>Vehicle info: <span class='match'>AEF 2348<\/span><\/li>\n<\/ul>"},"evals":[],"jsHooks":[]}</script>
---
## Repetition with quantifiers
Attached to literals or character classes, these allow a match to
repeat some number of times.
<br />
| Quantifier | Description |
|------------|-----------------|
| `*` | Match 0 or more |
| `+` | Match 1 or more |
| `?` | Match 0 or 1 |
| `{3}` | Match Exactly 3 |
| `{3,}` | Match 3 or more |
| `{3,5}` | Match 3, 4 or 5 |
---
## Examples: quantifiers
```r
text <- c("My", "cell: ", "(610)-867-5309")
```
--
```r
str_detect(text, "\\(\\d{3}\\)-\\d{3}-\\d{4}")
```
```
#> [1] FALSE FALSE TRUE
```
```r
str_extract(text, "\\(\\d{3}\\)-\\d{3}-\\d{4}")
```
```
#> [1] NA NA "(610)-867-5309"
```
--
```r
text <- "2 too two 4 for four 8 ate eight"
```
```r
str_extract(text, "\\d.*\\d")
```
--
```
#> [1] "2 too two 4 for four 8"
```
---
## Greedy matches
By default matches are greedy. This is why we get
```
#> [1] "2 too two 4 for four 8"
```
instead of
```
#> [1] "2 too two 4"
```
when we run code
```r
str_extract(text, "\\d.*\\d")
```
--
To make matching lazy, include `?` after so you return the shortest substring
possible.
```r
str_extract(text, "\\d.*?\\d")
```
```
#> [1] "2 too two 4"
```
--
What will this result be?
```r
str_extract_all(c("fruit flies", "fly faster"), "[aeiou]{1,2}[a-z]+")
```
---
## Groups
Groups allow you to connect pieces of a regular
expression for modification or capture.
```r
str_extract(c("grey", "gray", "gravitas", "great"), "gre|ay")
```
```
#> [1] "gre" "ay" NA "gre"
```
--
```r
str_extract(c("grey", "gray", "gravitas", "great"), "grey|gray")
```
```
#> [1] "grey" "gray" NA NA
```
--
```r
str_extract(c("grey", "gray", "gravitas", "great"), "gr(e|a)y")
```
```
#> [1] "grey" "gray" NA NA
```
<br/>
Their use can improve readability and allow for backreferencing.
---
## Capture groups
Suppose we have the following files, where want to capture their name and not
the file type.
```r
files <- c("dog.png", "cat44.png", "file_0292.png", "notes-v2.png")
```
--
```r
str_match(files, "(\\w+[[:punct:]]?\\w+)\\.png")
```
```
#> [,1] [,2]
#> [1,] "dog.png" "dog"
#> [2,] "cat44.png" "cat44"
#> [3,] "file_0292.png" "file_0292"
#> [4,] "notes-v2.png" "notes-v2"
```
--
Without the parentheses we get
```r
str_match(files, "\\w+[[:punct:]]?\\w+\\.png")
```
```
#> [,1]
#> [1,] "dog.png"
#> [2,] "cat44.png"
#> [3,] "file_0292.png"
#> [4,] "notes-v2.png"
```
---
## Backreferences
Backreferencing allows us to reference groups with `\1`, `\2`, etc.
```r
text <- "Some numbers include 00, 11, 3434, 41, 1010, 23, and 1"
```
--
```r
str_match_all(text, "(\\d)\\1")
```
```
#> [[1]]
#> [,1] [,2]
#> [1,] "00" "0"
#> [2,] "11" "1"
```
--
```r
str_match_all(text, "(\\d{2})\\1")
```
```
#> [[1]]
#> [,1] [,2]
#> [1,] "3434" "34"
#> [2,] "1010" "10"
```
---
## Exercises
```r
text <- c(
"apple",
"219 733 8965",
"329-293-8753",
"Work: (579) 499-7527; Home: (543) 355 3679"
)
```
1. Write a regular expression that will extract all phone numbers contained
in the vector above.
2. Once that works, use groups to extracts the area code separately from the rest
of the phone number.
???
## Solution
```r
str_extract_all(text, "\\(?\\d{3}\\)?[ -]\\d{3}[ -]\\d{4}")
```
```
#> [[1]]
#> character(0)
#>
#> [[2]]
#> [1] "219 733 8965"
#>
#> [[3]]
#> [1] "329-293-8753"
#>
#> [[4]]
#> [1] "(579) 499-7527" "(543) 355 3679"
```
```r
str_match_all(text, "\\(?(\\d{3})\\)?[ -]\\d{3}[ -]\\d{4}") %>%
purrr::map(~ .[, 2]) %>%
unlist()
```
```
#> [1] "219" "329" "579" "543"
```
---
## References
1. Grolemund, G., & Wickham, H. (2020). R for Data Science.
https://r4ds.had.co.nz/
2. Regular expressions. (2020). Stringr.tidyverse.org.
https://stringr.tidyverse.org/articles/regular-expressions.html
3. [Regular-Expression.info](http://www.regular-expressions.info/)
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