RegularExpressionPrimer < Archive < Vanderbilt Biostatistics Wiki

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Slide template topic Archive.BiostatSlideTemplate not found or empty!Regular expressions

Jeremy Stephens - Thursday, September 29, 2011

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Slide template topic Archive.BiostatSlideTemplate not found or empty!What are they?

Regular expressions are a way to describe patterns in text.

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Slide template topic Archive.BiostatSlideTemplate not found or empty!When to use them

Finding things
Replacing things: c("123", "123x", "abc123")
Extracting data:
"Tom Brady passed for 5 touchdowns and 456 yards, but they lost anyway."

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Slide template topic Archive.BiostatSlideTemplate not found or empty!Barriers to using regular expressions

Photo: Carole Pasquier; Regular expression: Thomas Dupont

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Slide template topic Archive.BiostatSlideTemplate not found or empty!Advice

GFX credit: Seddon

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Slide template topic Archive.BiostatSlideTemplate not found or empty!Things you (might) need to know

There are 3 main regular expression standards:

POSIX Basic Regular Expressions
POSIX Extended Regular Expressions (R uses this by default)
Perl-based Regular Expressions (R has support for this as well)

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Slide template topic Archive.BiostatSlideTemplate not found or empty!The simplest regular expression

Plain text!

> grep("test", c("foo", "bar", "test"))
[1] 3

"test" is a perfectly valid regular expression
Works just like CTRL+F in your browser

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Slide template topic Archive.BiostatSlideTemplate not found or empty!Metacharacters

Characters that have special meaning
Meta, meaning "about"

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Slide template topic Archive.BiostatSlideTemplate not found or empty!Your first metacharacter: Dot

Dot (or period) matches any character

> grep("foo.bar", c("fooxbar", "foo bar", "foobar", "foo12bar"))
[1] 1 2

"foo.bar" is the regular expression

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Slide template topic Archive.BiostatSlideTemplate not found or empty!Matching two wildcards

> grep("foo..bar", c("fooxbar", "foo bar", "foobar", "foo12bar"))
[1] 4

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Slide template topic Archive.BiostatSlideTemplate not found or empty!Variable length matching, using "plus"

> grep("foo.+bar", c("fooxbar", "foo bar", "foobar", "foo12bar"))
[1] 1 2 4

"foo.+bar" is the regular expression
The plus metacharacter means: "one or more times"

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Slide template topic Archive.BiostatSlideTemplate not found or empty!More "plus" usage

> grep("bar+", c("bar", "barr", "barrrrrrrr", "ba"))
[1] 1 2 3

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Slide template topic Archive.BiostatSlideTemplate not found or empty!Quantifiers

Metacharacters that describe "how many"
The "plus" metacharacter is a quantifier

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Slide template topic Archive.BiostatSlideTemplate not found or empty!Star metacharacter: "Zero or more times"

> grep("foo.*bar", c("fooxbar", "foo bar", "foobar", "foo12bar"))
[1] 1 2 3 4

> grep("bar*", c("bar", "barr", "barrrr", "ba"))
[1] 1 2 3 4

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Slide template topic Archive.BiostatSlideTemplate not found or empty!Question mark metacharacter: "Zero or one times"

> grep("abc?def", c("abdef", "abcdef", "abccdef"))
[1] 1 2

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Slide template topic Archive.BiostatSlideTemplate not found or empty!Brace metacharacter: "Exactly n times"

> grep("10{6}", c("1000", "1000000"))
[1] 2

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Slide template topic Archive.BiostatSlideTemplate not found or empty!Brace metacharacter improved

> grep("10{2,4}1", c("101", "1001", "10001", "100001", "1000001"))
[1] 2 3 4

{n, m} - Match at least n times, but not more than m times

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Slide template topic Archive.BiostatSlideTemplate not found or empty!Intermission

Questions?

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Slide template topic Archive.BiostatSlideTemplate not found or empty!Anchors!

> grep("10{3}", c("100", "1000", "10000"))
[1] 2 3

The third string matches!

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Slide template topic Archive.BiostatSlideTemplate not found or empty!Anchor to the end with a dollar sign

Dollar sign metacharacter:

> grep("10{3}$", c("100", "1000", "10000"))
[1] 2

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Slide template topic Archive.BiostatSlideTemplate not found or empty!Anchor to the beginning with a caret

> grep("10{3}", c("1000", "abc1000"))
[1] 1 2

Caret metacharacter:

> grep("^10{3}", c("1000", "abc1000"))
[1] 1

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Slide template topic Archive.BiostatSlideTemplate not found or empty!Using both anchors

> grep("10{3}", c("abc1000", "1000", "10000"))
[1] 1 2 3

> grep("10{3}$", c("abc1000", "1000", "10000"))
[1] 1 2

> grep("^10{3}$", c("abc1000", "1000", "10000"))
[1] 2

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Slide template topic Archive.BiostatSlideTemplate not found or empty!Character classes with brackets

In case you don't want to match any character by using dot:

> grep("ab[cdef]", c("abc", "abd", "abe", "abf", "abg")) 
[1] 1 2 3 4

Bracket metacharacter

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Slide template topic Archive.BiostatSlideTemplate not found or empty!Character class ranges

Instead of typing out all of those letters:

> grep("ab[c-f]", c("abc", "abd", "abe", "abf", "abg")) 
[1] 1 2 3 4

Dash metacharacter

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Slide template topic Archive.BiostatSlideTemplate not found or empty!Multiple ranges

> grep("ab[c-fC-F]", c("abc", "abC", "abf", "abF", "abg")) 
[1] 1 2 3 4

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Slide template topic Archive.BiostatSlideTemplate not found or empty!Mixing ranges and characters

> grep("ab[c-fC-F123]", c("abc", "abF", "ab1", "ab2")) 
[1] 1 2 3 4

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Slide template topic Archive.BiostatSlideTemplate not found or empty!Context matters

Because regular expressions aren't hard enough
Some metacharacters mean different things depending on context

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Slide template topic Archive.BiostatSlideTemplate not found or empty!Context in character classes

Most metacharacters lose their meaning inside brackets:

> grep("[.+*]", c(".", "+", "*", "x"))
[1] 1 2 3
> grep("[a-c-]", c("a", "b", "c", "-"))
[1] 1 2 3 4

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Slide template topic Archive.BiostatSlideTemplate not found or empty!Full circle

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Slide template topic Archive.BiostatSlideTemplate not found or empty!Better e-mail regular expression