4.8 Regular Expressions

Regular Expressions in The Racket Guide introduces regular expressions.

Regular expressions are specified as strings or byte strings, using the same pattern language as either the Unix utility egrep or Perl. A string-specified pattern produces a character regexp matcher, and a byte-string pattern produces a byte regexp matcher. If a character regexp is used with a byte string or input port, it matches UTF-8 encodings (see Encodings and Locales) of matching character streams; if a byte regexp is used with a character string, it matches bytes in the UTF-8 encoding of the string.

A regular expression that is represented as a string or byte string can be compiled to a regexp value, which can be used more efficiently by functions such as regexp-match compared to the string or byte string form. The regexp and byte-regexp procedures convert a string or byte string (respectively) into a regexp value using a syntax of regular expressions that is most compatible to egrep. The pregexp and byte-pregexp procedures produce a regexp value using a slightly different syntax of regular expressions that is more compatible with Perl.

Two regexp values are equal? if they have the same source, use the same pattern language, and are both character regexps or both byte regexps.

A literal or printed regexp value starts with #rx or #px. See Reading Regular Expressions for information on reading regular expressions and Printing Regular Expressions for information on printing regular expressions. Regexp values produced by the default reader are interned in read-syntax mode.

On the BC variant of Racket, the internal size of a regexp value is limited to 32 kilobytes; this limit roughly corresponds to a source string with 32,000 literal characters or 5,000 operators.

4.8.1 Regexp Syntax

The following syntax specifications describe the content of a string that represents a regular expression. The syntax of the corresponding string may involve extra escape characters. For example, the regular expression (.*)\1 can be represented with the string "(.*)\\1" or the regexp constant #rx"(.*)\\1"; the \ in the regular expression must be escaped to include it in a string or regexp constant.

The regexp and pregexp syntaxes share a common core:

‹regexp›	::=	‹pces›	Match ‹pces›
	\|	‹regexp›\|‹regexp›	Match either ‹regexp›, try left first	ex1
‹pces›	::=		Match empty
	\|	‹pce›‹pces›	Match ‹pce› followed by ‹pces›
‹pce›	::=	‹repeat›	Match ‹repeat›, longest possible	ex3
	\|	‹repeat›?	Match ‹repeat›, shortest possible	ex6
	\|	‹atom›	Match ‹atom› exactly once
‹repeat›	::=	‹atom›*	Match ‹atom› 0 or more times	ex3
	\|	‹atom›+	Match ‹atom› 1 or more times	ex4
	\|	‹atom›?	Match ‹atom› 0 or 1 times	ex5
‹atom›	::=	(‹regexp›)	Match sub-expression ‹regexp› and report	ex11
	\|	[‹rng›]	Match any character in ‹rng›	ex2
	\|	[^‹rng›]	Match any character not in ‹rng›	ex12
	\|	.	Match any (except newline in multi mode)	ex13
	\|	^	Match start (or after newline in multi mode)	ex14
	\|	$	Match end (or before newline in multi mode)	ex15
	\|	‹literal›	Match a single literal character	ex1
	\|	(?‹mode›:‹regexp›)	Match ‹regexp› using ‹mode›	ex35
	\|	(?>‹regexp›)	Match ‹regexp›, only first possible
	\|	‹look›	Match empty if ‹look› matches
	\|	(?‹tst›‹pces›\|‹pces›)	Match 1st ‹pces› if ‹tst›, else 2nd ‹pces›	ex36
	\|	(?‹tst›‹pces›)	Match ‹pces› if ‹tst›, empty if not ‹tst›
	\|	\ at end of pattern	Match the nul character (ASCII 0)
‹rng›	::=	]	‹rng› contains ] only	ex27
	\|	-	‹rng› contains - only	ex28
	\|	‹mrng›	‹rng› contains everything in ‹mrng›
	\|	‹mrng›-	‹rng› contains - and everything in ‹mrng›
‹mrng›	::=	]‹lrng›	‹mrng› contains ] and everything in ‹lrng›	ex29
	\|	-‹lrng›	‹mrng› contains - and everything in ‹lrng›	ex29
	\|	‹lirng›	‹mrng› contains everything in ‹lirng›
‹lirng›	::=	‹riliteral›	‹lirng› contains a literal character
	\|	‹riliteral›-‹rliteral›	‹lirng› contains Unicode range inclusive	ex22
	\|	‹lirng›‹lrng›	‹lirng› contains everything in both
‹lrng›	::=	^	‹lrng› contains ^	ex30
	\|	‹rliteral›-‹rliteral›	‹lrng› contains Unicode range inclusive
	\|	^‹lrng›	‹lrng› contains ^ and more
	\|	‹lirng›	‹lrng› contains everything in ‹lirng›
‹look›	::=	(?=‹regexp›)	Match if ‹regexp› matches	ex31
	\|	(?!‹regexp›)	Match if ‹regexp› doesn't match	ex32
	\|	(?<=‹regexp›)	Match if ‹regexp› matches preceding	ex33
	\|	(?<!‹regexp›)	Match if ‹regexp› doesn't match preceding	ex34
‹tst›	::=	(‹n›)	True if ‹n›th ( has a match
	\|	‹look›	True if ‹look› matches	ex36
‹mode›	::=		Like the enclosing mode
	\|	‹mode›i	Like ‹mode›, but case-insensitive	ex35
	\|	‹mode›-i	Like ‹mode›, but sensitive
	\|	‹mode›s	Like ‹mode›, but not in multi mode
	\|	‹mode›-s	Like ‹mode›, but in multi mode
	\|	‹mode›m	Like ‹mode›, but in multi mode
	\|	‹mode›-m	Like ‹mode›, but not in multi mode

The following completes the grammar for regexp, which treats { and } as literals, \ as a literal within ranges, and \ as a literal producer outside of ranges.

‹literal›	::=	Any character except (, ), *, +, ?, [, ., ^, \, or \|
	\|	\‹aliteral›	Match ‹aliteral›	ex21
‹aliteral›	::=	Any character
‹riliteral›	::=	Any character except ], -, or ^
‹rliteral›	::=	Any character except ] or -

The following completes the grammar for pregexp, which uses { and } bounded repetition and uses \ for meta-characters both inside and outside of ranges.

‹repeat›	::=	...	...
	\|	‹atom›{‹n›}	Match ‹atom› exactly ‹n› times	ex7
	\|	‹atom›{‹n›,}	Match ‹atom› ‹n› or more times	ex8
	\|	‹atom›{,‹m›}	Match ‹atom› between 0 and ‹m› times	ex9
	\|	‹atom›{‹n›,‹m›}	Match ‹atom› between ‹n› and ‹m› times	ex10
	\|	‹atom›{}	Match ‹atom› 0 or more times
‹atom›	::=	...	...
	\|	\‹n›	Match latest reported match for ‹n›th (	ex16
	\|	‹class›	Match any character in ‹class›
	\|	\b	Match \w* boundary	ex17
	\|	\B	Match where \b does not	ex18
	\|	\p{‹property›}	Match (UTF-8 encoded) in ‹property›	ex19
	\|	\P{‹property›}	Match (UTF-8 encoded) not in ‹property›	ex20
‹literal›	::=	Any character except (, ), *, +, ?, [, ], {, }, ., ^, \, or \|
	\|	\‹aliteral›	Match ‹aliteral›	ex21
‹aliteral›	::=	Any character except a-z, A-Z, 0-9
‹lirng›	::=	...	...
	\|	‹class›	‹lirng› contains all characters in ‹class›
	\|	‹posix›	‹lirng› contains all characters in ‹posix›	ex26
	\|	\‹eliteral›	‹lirng› contains ‹eliteral›
‹riliteral›	::=	Any character except ], \, -, or ^
‹rliteral›	::=	Any character except ], \, or -
‹eliteral›	::=	Any character except a-z, A-Z
‹class›	::=	\d	Contains 0-9	ex23
	\|	\D	Contains characters not in \d
	\|	\w	Contains a-z, A-Z, 0-9, _	ex24
	\|	\W	Contains characters not in \w
	\|	\s	Contains space, tab, newline, formfeed, return	ex25
	\|	\S	Contains characters not in \s
‹posix›	::=	[:alpha:]	Contains a-z, A-Z
	\|	[:upper:]	Contains A-Z
	\|	[:lower:]	Contains a-z	ex26
	\|	[:digit:]	Contains 0-9
	\|	[:xdigit:]	Contains 0-9, a-f, A-F
	\|	[:alnum:]	Contains a-z, A-Z, 0-9
	\|	[:word:]	Contains a-z, A-Z, 0-9, _
	\|	[:blank:]	Contains space and tab
	\|	[:space:]	Contains space, tab, newline, formfeed, return
	\|	[:graph:]	Contains all ASCII characters that use ink
	\|	[:print:]	Contains space, tab, and ASCII ink users
	\|	[:cntrl:]	Contains all characters with scalar value < 32
	\|	[:ascii:]	Contains all ASCII characters
‹property›	::=	‹category›	Includes all characters in ‹category›
	\|	^‹category›	Includes all characters not in ‹category›

In case-insensitive mode, a backreference of the form \‹n› matches case-insensitively only with respect to ASCII characters.

The Unicode categories follow.

‹category›	::=	Ll	Letter, lowercase	ex19
	\|	Lu	Letter, uppercase
	\|	Lt	Letter, titlecase
	\|	Lm	Letter, modifier
	\|	L&	Union of Ll, Lu, Lt, and Lm
	\|	Lo	Letter, other
	\|	L	Union of L& and Lo
	\|	Nd	Number, decimal digit
	\|	Nl	Number, letter
	\|	No	Number, other
	\|	N	Union of Nd, Nl, and No
	\|	Ps	Punctuation, open
	\|	Pe	Punctuation, close
	\|	Pi	Punctuation, initial quote
	\|	Pf	Punctuation, final quote
	\|	Pc	Punctuation, connector
	\|	Pd	Punctuation, dash
	\|	Po	Punctuation, other
	\|	P	Union of Ps, Pe, Pi, Pf, Pc, Pd, and Po
	\|	Mn	Mark, non-spacing
	\|	Mc	Mark, spacing combining
	\|	Me	Mark, enclosing
	\|	M	Union of Mn, Mc, and Me
	\|	Sc	Symbol, currency
	\|	Sk	Symbol, modifier
	\|	Sm	Symbol, math
	\|	So	Symbol, other
	\|	S	Union of Sc, Sk, Sm, and So
	\|	Zl	Separator, line
	\|	Zp	Separator, paragraph
	\|	Zs	Separator, space
	\|	Z	Union of Zl, Zp, and Zs
	\|	Cc	Other, control
	\|	Cf	Other, format
	\|	Cs	Other, surrogate
	\|	Cn	Other, not assigned
	\|	Co	Other, private use
	\|	C	Union of Cc, Cf, Cs, Cn, and Co
	\|	.	Union of all Unicode categories

Examples:

> (regexp-match #rx"a|b" "cat") ; ex1
'("a")
> (regexp-match #rx"[at]" "cat") ; ex2
'("a")
> (regexp-match #rx"ca*[at]" "caaat") ; ex3
'("caaat")
> (regexp-match #rx"ca+[at]" "caaat") ; ex4
'("caaat")
> (regexp-match #rx"ca?t?" "ct") ; ex5
'("ct")
> (regexp-match #rx"ca*?[at]" "caaat") ; ex6
'("ca")
> (regexp-match #px"ca{2}" "caaat") ; ex7, uses #px
'("caa")
> (regexp-match #px"ca{2,}t" "catcaat") ; ex8, uses #px
'("caat")
> (regexp-match #px"ca{,2}t" "caaatcat") ; ex9, uses #px
'("cat")
> (regexp-match #px"ca{1,2}t" "caaatcat") ; ex10, uses #px
'("cat")
> (regexp-match #rx"(c*)(a*)" "caat") ; ex11
'("caa" "c" "aa")
> (regexp-match #rx"[^ca]" "caat") ; ex12
'("t")
> (regexp-match #rx".(.)." "cat") ; ex13
'("cat" "a")
> (regexp-match #rx"^a|^c" "cat") ; ex14
'("c")
> (regexp-match #rx"a$|t$" "cat") ; ex15
'("t")
> (regexp-match #px"c(.)\\1t" "caat") ; ex16, uses #px
'("caat" "a")
> (regexp-match #px".\\b." "cat in hat") ; ex17, uses #px
'("t ")
> (regexp-match #px".\\B." "cat in hat") ; ex18, uses #px
'("ca")
> (regexp-match #px"\\p{Ll}" "Cat") ; ex19, uses #px
'("a")
> (regexp-match #px"\\P{Ll}" "cat!") ; ex20, uses #px
'("!")
> (regexp-match #rx"\\|" "c|t") ; ex21
'("|")
> (regexp-match #rx"[a-f]*" "cat") ; ex22
'("ca")
> (regexp-match #px"[a-f\\d]*" "1cat") ; ex23, uses #px
'("1ca")
> (regexp-match #px" [\\w]" "cat hat") ; ex24, uses #px
'(" h")
> (regexp-match #px"t[\\s]" "cat\nhat") ; ex25, uses #px
'("t\n")
> (regexp-match #px"[[:lower:]]+" "Cat") ; ex26, uses #px
'("at")
> (regexp-match #rx"[]]" "c]t") ; ex27
'("]")
> (regexp-match #rx"[-]" "c-t") ; ex28
'("-")
> (regexp-match #rx"[]a[]+" "c[a]t") ; ex29
'("[a]")
> (regexp-match #rx"[a^]+" "ca^t") ; ex30
'("a^")
> (regexp-match #rx".a(?=p)" "cat nap") ; ex31
'("na")
> (regexp-match #rx".a(?!t)" "cat nap") ; ex32
'("na")
> (regexp-match #rx"(?<=n)a." "cat nap") ; ex33
'("ap")
> (regexp-match #rx"(?<!c)a." "cat nap") ; ex34
'("ap")
> (regexp-match #rx"(?i:a)[tp]" "cAT nAp") ; ex35
'("Ap")
> (regexp-match #rx"(?(?<=c)a|b)+" "cabal") ; ex36
'("ab")

4.8.2 Additional Syntactic Constraints

In addition to matching a grammar, regular expressions must meet two syntactic restrictions:

In a ‹repeat› other than ‹atom›?, the ‹atom› must not match an empty sequence.
In a (?<=‹regexp›) or (?<!‹regexp›), the ‹regexp› must match a bounded sequence only.

These constraints are checked syntactically by the following type system. A type [n, m] corresponds to an expression that matches between n and m characters. In the rule for (‹Regexp›), N means the number such that the opening parenthesis is the Nth opening parenthesis for collecting match reports. Non-emptiness is inferred for a backreference pattern, \‹N›, so that a backreference can be used for repetition patterns; in the case of mutual dependencies among backreferences, the inference chooses the fixpoint that maximizes non-emptiness. Finiteness is not inferred for backreferences (i.e., a backreference is assumed to match an arbitrarily large sequence).

‹regexp›1 : [n1, m1] ‹regexp›2 : [n2, m2]

‹regexp›1|‹regexp›2 : [min(n1, n2), max(m1, m2)]

‹pce› : [n1, m1] ‹pces› : [n2, m2]

‹pce›‹pces› : [n1+n2, m1+m2]

‹repeat› : [n, m]

‹repeat›? : [0, m]

‹atom› : [n, m] n > 0

‹atom›* : [0, ∞]

‹atom› : [n, m] n > 0

‹atom›+ : [1, ∞]

‹atom› : [n, m]

‹atom›? : [0, m]

‹atom› : [n, m] n > 0

‹atom›{‹n›} : [n*‹n›, m*‹n›]

‹atom› : [n, m] n > 0

‹atom›{‹n›,} : [n*‹n›, ∞]

‹atom› : [n, m] n > 0

‹atom›{,‹m›} : [0, m*‹m›]

‹atom› : [n, m] n > 0

‹atom›{‹n›,‹m›} : [n*‹n›, m*‹m›]

‹regexp› : [n, m]

(‹regexp›) : [n, m] αN=n

‹regexp› : [n, m]

(?‹mode›:‹regexp›) : [n, m]

‹regexp› : [n, m]

(?=‹regexp›) : [0, 0]

‹regexp› : [n, m]

(?!‹regexp›) : [0, 0]

‹regexp› : [n, m] m < ∞

(?<=‹regexp›) : [0, 0]

‹regexp› : [n, m] m < ∞

(?<!‹regexp›) : [0, 0]

‹regexp› : [n, m]

(?>‹regexp›) : [n, m]

‹tst› : [n0, m0] ‹pces›1 : [n1, m1] ‹pces›2 : [n2, m2]

(?‹tst›‹pces›1|‹pces›2) : [min(n1, n2), max(m1, m2)]

‹tst› : [n0, m0] ‹pces› : [n1, m1]

(?‹tst›‹pces›) : [0, m1]

(‹n›) : [αN, ∞]

[‹rng›] : [1, 1]

[^‹rng›] : [1, 1]

. : [1, 1]

^ : [0, 0]

$ : [0, 0]

‹literal› : [1, 1]

\‹n› : [αN, ∞]

‹class› : [1, 1]

\b : [0, 0]

\B : [0, 0]

\p{‹property›} : [1, 6]

\P{‹property›} : [1, 6]

4.8.3 Regexp Constructors

procedure
(regexp? v) → boolean?
v : any/c

Returns #t if v is a regexp value created by regexp or pregexp, #f otherwise.

procedure
(pregexp? v) → boolean?
v : any/c

Returns #t if v is a regexp value created by pregexp (not regexp), #f otherwise.

procedure
(byte-regexp? v) → boolean?
v : any/c

Returns #t if v is a regexp value created by byte-regexp or byte-pregexp, #f otherwise.

procedure
(byte-pregexp? v) → boolean?
v : any/c

Returns #t if v is a regexp value created by byte-pregexp (not byte-regexp), #f otherwise.

procedure
(regexp str) → regexp?
  str : string?
(regexp str handler) → any
  str : string?
  handler : (or/c #f (string? -> any))

Takes a string representation of a regular expression (using the syntax in Regexp Syntax) and compiles it into a regexp value. Other regular expression procedures accept either a string or a regexp value as the matching pattern. If a regular expression string is used multiple times, it is faster to compile the string once to a regexp value and use it for repeated matches instead of using the string each time.

If handler is provided and not #f, it is called and its result is returned when str is not a valid representation of a regular expression; the argument to handler is a string that describes the problem with str. If handler is #f or not provided, then exn:fail:contract exception is raised.

The object-name procedure returns the source string for a regexp value.

Examples:

> (regexp "ap*le")
#rx"ap*le"
> (object-name #rx"ap*le")
"ap*le"
> (regexp "+" (λ (s) (list s)))
'("`+` follows nothing in pattern")

Changed in version 6.5.0.1 of package base: Added the handler argument.

procedure
(pregexp str) → pregexp?
  str : string?
(pregexp str handler) → any
  str : string?
  handler : (or/c #f (string? -> any))

Like regexp, except that it uses a slightly different syntax (see Regexp Syntax). The result can be used with regexp-match, etc., just like the result from regexp.

Examples:

> (pregexp "ap*le")
#px"ap*le"
> (regexp? #px"ap*le")
#t
> (pregexp "+" (λ (s) (vector s)))
'#("`+` follows nothing in pattern")

Changed in version 6.5.0.1 of package base: Added the handler argument.

procedure
(byte-regexp bstr) → byte-regexp?
  bstr : bytes?
(byte-regexp bstr handler) → any
  bstr : bytes?
  handler : (or/c #f (bytes? -> any))

Takes a byte-string representation of a regular expression (using the syntax in Regexp Syntax) and compiles it into a byte-regexp value.

If handler is provided, it is called and its result is returned if str is not a valid representation of a regular expression.

The object-name procedure returns the source byte string for a regexp value.

Examples:

> (byte-regexp #"ap*le")
#rx#"ap*le"
> (object-name #rx#"ap*le")
#"ap*le"
> (byte-regexp "ap*le")
byte-regexp: contract violation
expected: bytes?
given: "ap*le"
> (byte-regexp #"+" (λ (s) (list s)))
'("`+` follows nothing in pattern")

Changed in version 6.5.0.1 of package base: Added the handler argument.

procedure
(byte-pregexp bstr) → byte-pregexp?
  bstr : bytes?
(byte-pregexp bstr handler) → any
  bstr : bytes?
  handler : (or/c #f (bytes? -> any))

Like byte-regexp, except that it uses a slightly different syntax (see Regexp Syntax). The result can be used with regexp-match, etc., just like the result from byte-regexp.

Examples:

> (byte-pregexp #"ap*le")
#px#"ap*le"
> (byte-pregexp #"+" (λ (s) (vector s)))
'#("`+` follows nothing in pattern")

Changed in version 6.5.0.1 of package base: Added the handler argument.

procedure
(regexp-quote str [case-sensitive?]) → string?
  str : string?
  case-sensitive? : any/c = #t
(regexp-quote bstr [case-sensitive?]) → bytes?
  bstr : bytes?
  case-sensitive? : any/c = #t

Produces a string or byte string suitable for use with regexp to match the literal sequence of characters in str or sequence of bytes in bstr. If case-sensitive? is true (the default), the resulting regexp matches letters in str or bytes case-sensitively, otherwise it matches case-insensitively.

Examples:

> (regexp-match "." "apple.scm")
'("a")
> (regexp-match (regexp-quote ".") "apple.scm")
'(".")

procedure
(regexp-max-lookbehind pattern) → exact-nonnegative-integer?
pattern : (or/c regexp? byte-regexp?)

Returns the maximum number of bytes that pattern may consult before the starting position of a match to determine the match. For example, the pattern (?<=abc)d consults three bytes preceding a matching d, while e(?<=a..)d consults two bytes before a matching ed. A ^ pattern may consult a preceding byte to determine whether the current position is the start of the input or of a line.

4.8.4 Regexp Matching

procedure
(regexp-match pattern
input
[ start-pos
end-pos
output-port
input-prefix])
→
(if (and (or (string? pattern) (regexp? pattern))
         (or (string? input) (path? input)))
    (or/c #f (cons/c string? (listof (or/c string? #f))))
    (or/c #f (cons/c bytes?  (listof (or/c bytes?  #f)))))
  pattern : (or/c string? bytes? regexp? byte-regexp?)
  input : (or/c string? bytes? path? input-port?)
  start-pos : exact-nonnegative-integer? = 0
  end-pos : (or/c exact-nonnegative-integer? #f) = #f
  output-port : (or/c output-port? #f) = #f
  input-prefix : bytes? = #""

Attempts to match pattern (a string, byte string, regexp value, or byte-regexp value) once to a portion of input. The matcher finds a portion of input that matches and is closest to the start of the input (after start-pos).

If input is a path, it is converted to a byte string with path->bytes if pattern is a byte string or a byte-based regexp. Otherwise, input is converted to a string with path->string.

The optional start-pos and end-pos arguments select a portion of input for matching; the default is the entire string or the stream up to an end-of-file. When input is a string, start-pos is a character position; when input is a byte string, then start-pos is a byte position; and when input is an input port, start-pos is the number of bytes to skip before starting to match. The end-pos argument can be #f, which corresponds to the end of the string or an end-of-file in the stream; otherwise, it is a character or byte position, like start-pos. If input is an input port, and if an end-of-file is reached before start-pos bytes are skipped, then the match fails.

In pattern, a start-of-string ^ refers to the first position of input after start-pos, assuming that input-prefix is #"". The end-of-input $ refers to the end-posth position or (in the case of an input port) an end-of-file, whichever comes first.

The input-prefix specifies bytes that effectively precede input for the purposes of ^ and other look-behind matching. For example, a #"" prefix means that ^ matches at the beginning of the stream, while a #"\n" input-prefix means that a start-of-line ^ can match the beginning of the input, while a start-of-file ^ cannot.

If the match fails, #f is returned. If the match succeeds, a list containing strings or byte string, and possibly #f, is returned. The list contains strings only if input is a string and pattern is not a byte regexp. Otherwise, the list contains byte strings (substrings of the UTF-8 encoding of input, if input is a string).

The first [byte] string in a result list is the portion of input that matched pattern. If two portions of input can match pattern, then the match that starts earliest is found.

Additional [byte] strings are returned in the list if pattern contains parenthesized sub-expressions (but not when the opening parenthesis is followed by ?). Matches for the sub-expressions are provided in the order of the opening parentheses in pattern. When sub-expressions occur in branches of an | “or” pattern, in a * “zero or more” pattern, or other places where the overall pattern can succeed without a match for the sub-expression, then a #f is returned for the sub-expression if it did not contribute to the final match. When a single sub-expression occurs within a * “zero or more” pattern or other multiple-match positions, then the rightmost match associated with the sub-expression is returned in the list.

If the optional output-port is provided as an output port, the part of input from its beginning (not start-pos) that precedes the match is written to the port. All of input up to end-pos is written to the port if no match is found. This functionality is most useful when input is an input port.

When matching an input port, a match failure reads up to end-pos bytes (or end-of-file), even if pattern begins with a start-of-string ^; see also regexp-try-match. On success, all bytes up to and including the match are eventually read from the port, but matching proceeds by first peeking bytes from the port (using peek-bytes-avail!), and then (re‑)reading matching bytes to discard them after the match result is determined. Non-matching bytes may be read and discarded before the match is determined. The matcher peeks in blocking mode only as far as necessary to determine a match, but it may peek extra bytes to fill an internal buffer if immediately available (i.e., without blocking). Greedy repeat operators in pattern, such as * or +, tend to force reading the entire content of the port (up to end-pos) to determine a match.

If the input port is read simultaneously by another thread, or if the port is a custom port with inconsistent reading and peeking procedures (see Custom Ports), then the bytes that are peeked and used for matching may be different than the bytes read and discarded after the match completes; the matcher inspects only the peeked bytes. To avoid such interleaving, use regexp-match-peek (with a progress-evt argument) followed by port-commit-peeked.

Examples:

> (regexp-match #rx"x." "12x4x6")
'("x4")
> (regexp-match #rx"y." "12x4x6")
#f
> (regexp-match #rx"x." "12x4x6" 3)
'("x6")
> (regexp-match #rx"x." "12x4x6" 3 4)
#f
> (regexp-match #rx#"x." "12x4x6")
'(#"x4")
> (regexp-match #rx"x." "12x4x6" 0 #f (current-output-port))
12
'("x4")
> (regexp-match #rx"(-[0-9]*)+" "a-12--345b")
'("-12--345" "-345")

procedure
(regexp-match* pattern
input
[ start-pos
end-pos
input-prefix
#:match-select match-select
#:gap-select? gap-select])
→
(if (and (or (string? pattern) (regexp? pattern))
         (or (string? input) (path? input)))
    (listof (or/c string? (listof (or/c #f string?))))
    (listof (or/c bytes? (listof (or/c #f bytes?)))))
  pattern : (or/c string? bytes? regexp? byte-regexp?)
  input : (or/c string? bytes? path? input-port?)
  start-pos : exact-nonnegative-integer? = 0
  end-pos : (or/c exact-nonnegative-integer? #f) = #f
  input-prefix : bytes? = #""
   match-select :
(or/c (list? . -> . (or/c any/c list?))
      #f)
= car
  gap-select : any/c = #f

Like regexp-match, but the result is a list of strings or byte strings corresponding to a sequence of matches of pattern in input.

The pattern is used in order to find matches, where each match attempt starts at the end of the last match, and ^ is allowed to match the beginning of the input (if input-prefix is #"") only for the first match. Empty matches are handled like other matches, returning a zero-length string or byte sequence (they are more useful in making this a complement of regexp-split), but pattern is restricted from matching an empty sequence immediately after an empty match.

If input contains no matches (in the range start-pos to end-pos), null is returned. Otherwise, each item in the resulting list is a distinct substring or byte sequence from input that matches pattern. The end-pos argument can be #f to match to the end of input (which corresponds to an end-of-file if input is an input port).

Examples:

> (regexp-match* #rx"x." "12x4x6")
'("x4" "x6")
> (regexp-match* #rx"x*" "12x4x6")
'("" "" "x" "" "x" "" "")

match-select specifies the collected results. The default of car means that the result is the list of matches without returning parenthesized sub-patterns. It can be given as a ‘selector’ function which chooses an item from a list, or it can choose a list of items. For example, you can use cdr to get a list of lists of parenthesized sub-patterns matches, or values (as an identity function) to get the full matches as well. (Note that the selector must choose an element of its input list or a list of elements, but it must not inspect its input as they can be either a list of strings or a list of position pairs. Furthermore, the selector must be consistent in its choice(s).)

Examples:

> (regexp-match* #rx"x(.)" "12x4x6" #:match-select cadr)
'("4" "6")
> (regexp-match* #rx"x(.)" "12x4x6" #:match-select values)
'(("x4" "4") ("x6" "6"))

In addition, specifying gap-select as a non-#f value will make the result an interleaved list of the matches as well as the separators between them matches, starting and ending with a separator. In this case, match-select can be given as #f to return only the separators, making such uses equivalent to regexp-split.

Examples:

> (regexp-match* #rx"x(.)" "12x4x6" #:match-select cadr #:gap-select? #t)
'("12" "4" "" "6" "")
> (regexp-match* #rx"x(.)" "12x4x6" #:match-select #f #:gap-select? #t)
'("12" "" "")

procedure
(regexp-try-match pattern
input
[ start-pos
end-pos
output-port
input-prefix])
→ (or/c #f (cons/c bytes? (listof (or/c bytes? #f))))
  pattern : (or/c string? bytes? regexp? byte-regexp?)
  input : input-port?
  start-pos : exact-nonnegative-integer? = 0
  end-pos : (or/c exact-nonnegative-integer? #f) = #f
  output-port : (or/c output-port? #f) = #f
  input-prefix : bytes? = #""

Like regexp-match on input ports, except that if the match fails, no characters are read and discarded from in.

This procedure is especially useful with a pattern that begins with a start-of-string ^ or with a non-#f end-pos, since each limits the amount of peeking into the port. Otherwise, beware that a large portion of the stream may be peeked (and therefore pulled into memory) before the match succeeds or fails.

procedure
(regexp-match-positions pattern
input
[ start-pos
end-pos
output-port
input-prefix])
→
(or/c (cons/c (cons/c exact-nonnegative-integer?
                      exact-nonnegative-integer?)
              (listof (or/c (cons/c exact-nonnegative-integer?
                                    exact-nonnegative-integer?)
                            #f)))
      #f)
  pattern : (or/c string? bytes? regexp? byte-regexp?)
  input : (or/c string? bytes? path? input-port?)
  start-pos : exact-nonnegative-integer? = 0
  end-pos : (or/c exact-nonnegative-integer? #f) = #f
  output-port : (or/c output-port? #f) = #f
  input-prefix : bytes? = #""

Like regexp-match, but returns a list of number pairs (and #f) instead of a list of strings. Each pair of numbers refers to a range of characters or bytes in input. If the result for the same arguments with regexp-match would be a list of byte strings, the resulting ranges correspond to byte ranges; in that case, if input is a character string, the byte ranges correspond to bytes in the UTF-8 encoding of the string.

Range results are returned in a substring- and subbytes-compatible manner, independent of start-pos. In the case of an input port, the returned positions indicate the number of bytes that were read, including start-pos, before the first matching byte.

Examples:

> (regexp-match-positions #rx"x." "12x4x6")
'((2 . 4))
> (regexp-match-positions #rx"x." "12x4x6" 3)
'((4 . 6))
> (regexp-match-positions #rx"(-[0-9]*)+" "a-12--345b")
'((1 . 9) (5 . 9))

procedure
(regexp-match-positions* pattern
input
[ start-pos
end-pos
input-prefix
#:match-select match-select])
→
(or/c (listof (cons/c exact-nonnegative-integer?
                      exact-nonnegative-integer?))
      (listof (listof (or/c #f (cons/c exact-nonnegative-integer?
                                       exact-nonnegative-integer?)))))
  pattern : (or/c string? bytes? regexp? byte-regexp?)
  input : (or/c string? bytes? path? input-port?)
  start-pos : exact-nonnegative-integer? = 0
  end-pos : (or/c exact-nonnegative-integer? #f) = #f
  input-prefix : bytes? = #""
  match-select : (list? . -> . (or/c any/c list?)) = car

Like regexp-match-positions, but returns multiple matches like regexp-match*.

Examples:

> (regexp-match-positions* #rx"x." "12x4x6")
'((2 . 4) (4 . 6))
> (regexp-match-positions* #rx"x(.)" "12x4x6" #:match-select cadr)
'((3 . 4) (5 . 6))

Note that unlike regexp-match*, there is no #:gap-select? input keyword, as this information can be easily inferred from the resulting matches.

procedure
(regexp-match? pattern
input
[ start-pos
end-pos
output-port
input-prefix]) → boolean?
  pattern : (or/c string? bytes? regexp? byte-regexp?)
  input : (or/c string? bytes? path? input-port?)
  start-pos : exact-nonnegative-integer? = 0
  end-pos : (or/c exact-nonnegative-integer? #f) = #f
  output-port : (or/c output-port? #f) = #f
  input-prefix : bytes? = #""

Like regexp-match, but returns merely #t when the match succeeds, #f otherwise.

Examples:

> (regexp-match? #rx"x." "12x4x6")
#t
> (regexp-match? #rx"y." "12x4x6")
#f

procedure
(regexp-match-exact? pattern input) → boolean?
pattern : (or/c string? bytes? regexp? byte-regexp?)
input : (or/c string? bytes? path?)

Like regexp-match?, but #t is only returned when the entire content of input matches pattern.

Examples:

> (regexp-match-exact? #rx"x." "12x4x6")
#f
> (regexp-match-exact? #rx"1.*x." "12x4x6")
#t

procedure
(regexp-match-peek pattern
input
[ start-pos
end-pos
progress
input-prefix])
→
(or/c (cons/c bytes? (listof (or/c bytes? #f)))
      #f)
  pattern : (or/c string? bytes? regexp? byte-regexp?)
  input : input-port?
  start-pos : exact-nonnegative-integer? = 0
  end-pos : (or/c exact-nonnegative-integer? #f) = #f
  progress : (or/c evt #f) = #f
  input-prefix : bytes? = #""

Like regexp-match on input ports, but only peeks bytes from input instead of reading them. Furthermore, instead of an output port, the last optional argument is a progress event for input (see port-progress-evt). If progress becomes ready, then the match stops peeking from input and returns #f. The progress argument can be #f, in which case the peek may continue with inconsistent information if another process meanwhile reads from input.

Examples:

> (define p (open-input-string "a abcd"))
> (regexp-match-peek ".*bc" p)
'(#"a abc")
> (regexp-match-peek ".*bc" p 2)
'(#"abc")
> (regexp-match ".*bc" p 2)
'(#"abc")
> (peek-char p)
#\d
> (regexp-match ".*bc" p)
#f
> (peek-char p)
#<eof>

procedure
(regexp-match-peek-positions pattern
input
[ start-pos
end-pos
progress
input-prefix])
→
(or/c (cons/c (cons/c exact-nonnegative-integer?
                      exact-nonnegative-integer?)
              (listof (or/c (cons/c exact-nonnegative-integer?
                                    exact-nonnegative-integer?)
                            #f)))
      #f)
  pattern : (or/c string? bytes? regexp? byte-regexp?)
  input : input-port?
  start-pos : exact-nonnegative-integer? = 0
  end-pos : (or/c exact-nonnegative-integer? #f) = #f
  progress : (or/c evt #f) = #f
  input-prefix : bytes? = #""

Like regexp-match-positions on input ports, but only peeks bytes from input instead of reading them, and with a progress argument like regexp-match-peek.

procedure
(regexp-match-peek-immediate pattern
input
[ start-pos
end-pos
progress
input-prefix])
→
(or/c (cons/c bytes? (listof (or/c bytes? #f)))
      #f)
  pattern : (or/c string? bytes? regexp? byte-regexp?)
  input : input-port?
  start-pos : exact-nonnegative-integer? = 0
  end-pos : (or/c exact-nonnegative-integer? #f) = #f
  progress : (or/c evt #f) = #f
  input-prefix : bytes? = #""

Like regexp-match-peek, but it attempts to match only bytes that are available from input without blocking. The match fails if not-yet-available characters might be used to match pattern.

procedure
(regexp-match-peek-positions-immediate pattern
input
[ start-pos
end-pos
progress
input-prefix])
→
(or/c (cons/c (cons/c exact-nonnegative-integer?
                      exact-nonnegative-integer?)
              (listof (or/c (cons/c exact-nonnegative-integer?
                                    exact-nonnegative-integer?)
                            #f)))
      #f)
  pattern : (or/c string? bytes? regexp? byte-regexp?)
  input : input-port?
  start-pos : exact-nonnegative-integer? = 0
  end-pos : (or/c exact-nonnegative-integer? #f) = #f
  progress : (or/c evt #f) = #f
  input-prefix : bytes? = #""

Like regexp-match-peek-positions, but it attempts to match only bytes that are available from input without blocking. The match fails if not-yet-available characters might be used to match pattern.

procedure
(regexp-match-peek-positions* pattern
input
[ start-pos
end-pos
input-prefix
#:match-select match-select])
→
(or/c (listof (cons/c exact-nonnegative-integer?
                      exact-nonnegative-integer?))
      (listof (listof (or/c #f (cons/c exact-nonnegative-integer?
                                       exact-nonnegative-integer?)))))
  pattern : (or/c string? bytes? regexp? byte-regexp?)
  input : input-port?
  start-pos : exact-nonnegative-integer? = 0
  end-pos : (or/c exact-nonnegative-integer? #f) = #f
  input-prefix : bytes? = #""
  match-select : (list? . -> . (or/c any/c list?)) = car

Like regexp-match-peek-positions, but returns multiple matches like regexp-match-positions*.

procedure
(regexp-match/end pattern
input
[ start-pos
end-pos
output-port
input-prefix
count])
→
(if (and (or (string? pattern) (regexp? pattern))
         (or/c (string? input) (path? input)))
    (or/c #f (cons/c string? (listof (or/c string? #f))))
    (or/c #f (cons/c bytes?  (listof (or/c bytes?  #f)))))
(or/c #f bytes?)
  pattern : (or/c string? bytes? regexp? byte-regexp?)
  input : (or/c string? bytes? path? input-port?)
  start-pos : exact-nonnegative-integer? = 0
  end-pos : (or/c exact-nonnegative-integer? #f) = #f
  output-port : (or/c output-port? #f) = #f
  input-prefix : bytes? = #""
  count : exact-nonnegative-integer? = 1

Like regexp-match, but with a second result: a byte string of up to count bytes that correspond to the input (possibly including the input-prefix) leading to the end of the match; the second result is #f if no match is found.

The second result can be useful as an input-prefix for attempting a second match on input starting from the end of the first match. In that case, use regexp-max-lookbehind to determine an appropriate value for count.

procedure
(regexp-match-positions/end pattern
input
[ start-pos
end-pos
input-prefix
count])
→
(listof (cons/c exact-nonnegative-integer?
                exact-nonnegative-integer?))
(or/c #f bytes?)
  pattern : (or/c string? bytes? regexp? byte-regexp?)
  input : (or/c string? bytes? path? input-port?)
  start-pos : exact-nonnegative-integer? = 0
  end-pos : (or/c exact-nonnegative-integer? #f) = #f
  input-prefix : bytes? = #""
  count : exact-nonnegative-integer? = 1
procedure
(regexp-match-peek-positions/end pattern
input
[ start-pos
end-pos
progress
input-prefix
count])
→
(or/c (cons/c (cons/c exact-nonnegative-integer?
                      exact-nonnegative-integer?)
              (listof (or/c (cons/c exact-nonnegative-integer?
                                    exact-nonnegative-integer?)
                            #f)))
      #f)
(or/c #f bytes?)
  pattern : (or/c string? bytes? regexp? byte-regexp?)
  input : input-port?
  start-pos : exact-nonnegative-integer? = 0
  end-pos : (or/c exact-nonnegative-integer? #f) = #f
  progress : (or/c evt #f) = #f
  input-prefix : bytes? = #""
  count : exact-nonnegative-integer? = 1
procedure
(regexp-match-peek-positions-immediate/end pattern
input
[ start-pos
end-pos
progress
input-prefix
count])
→
(or/c (cons/c (cons/c exact-nonnegative-integer?
                      exact-nonnegative-integer?)
              (listof (or/c (cons/c exact-nonnegative-integer?
                                    exact-nonnegative-integer?)
                            #f)))
      #f)
(or/c #f bytes?)
  pattern : (or/c string? bytes? regexp? byte-regexp?)
  input : input-port?
  start-pos : exact-nonnegative-integer? = 0
  end-pos : (or/c exact-nonnegative-integer? #f) = #f
  progress : (or/c evt #f) = #f
  input-prefix : bytes? = #""
  count : exact-nonnegative-integer? = 1

Like regexp-match-positions, etc., but with a second result like regexp-match/end.

4.8.5 Regexp Splitting

procedure
(regexp-split pattern
input
[ start-pos
end-pos
input-prefix])
→
(if (and (or (string? pattern) (regexp? pattern))
         (string? input))
    (cons/c string? (listof string?))
    (cons/c bytes? (listof bytes?)))
  pattern : (or/c string? bytes? regexp? byte-regexp?)
  input : (or/c string? bytes? input-port?)
  start-pos : exact-nonnegative-integer? = 0
  end-pos : (or/c exact-nonnegative-integer? #f) = #f
  input-prefix : bytes? = #""

The complement of regexp-match*: the result is a list of strings (if pattern is a string or character regexp and input is a string) or byte strings (otherwise) from input that are separated by matches to pattern. Adjacent matches are separated with "" or #"". Zero-length matches are treated the same as for regexp-match*.

If input contains no matches (in the range start-pos to end-pos), the result is a list containing input’s content (from start-pos to end-pos) as a single element. If a match occurs at the beginning of input (at start-pos), the resulting list will start with an empty string or byte string, and if a match occurs at the end (at end-pos), the list will end with an empty string or byte string. The end-pos argument can be #f, in which case splitting goes to the end of input (which corresponds to an end-of-file if input is an input port).

Examples:

> (regexp-split #rx" +" "12  34")
'("12" "34")
> (regexp-split #rx"." "12  34")
'("" "" "" "" "" "" "")
> (regexp-split #rx"" "12  34")
'("" "1" "2" " " " " "3" "4" "")
> (regexp-split #rx" *" "12  34")
'("" "1" "2" "" "3" "4" "")
> (regexp-split #px"\\b" "12, 13 and 14.")
'("" "12" ", " "13" " " "and" " " "14" ".")
> (regexp-split #rx" +" "")
'("")

4.8.6 Regexp Substitution

procedure
(regexp-replace pattern
input
insert
[ input-prefix])
→
(if (and (or (string? pattern) (regexp? pattern))
         (string? input))
    string?
    bytes?)
  pattern : (or/c string? bytes? regexp? byte-regexp?)
  input : (or/c string? bytes?)
   insert :
(or/c string? bytes?
      ((string?) () #:rest (listof string?) . ->* . string?)
      ((bytes?) () #:rest (listof bytes?) . ->* . bytes?))
  input-prefix : bytes? = #""

Performs a match using pattern on input, and then returns a string or byte string in which the matching portion of input is replaced with insert. If pattern matches no part of input, then input is returned unmodified.

The insert argument can be either a (byte) string, or a function that returns a (byte) string. In the latter case, the function is applied on the list of values that regexp-match would return (i.e., the first argument is the complete match, and then one argument for each parenthesized sub-expression) to obtain a replacement (byte) string.

If pattern is a string or character regexp and input is a string, then insert must be a string or a procedure that accept strings, and the result is a string. If pattern is a byte string or byte regexp, or if input is a byte string, then insert as a string is converted to a byte string, insert as a procedure is called with a byte string, and the result is a byte string.

If insert contains &, then & is replaced with the matching portion of input before it is substituted into the match’s place. If insert contains \‹n› for some integer ‹n›, then it is replaced with the ‹n›th matching sub-expression from input. A & and \0 are aliases. If the ‹n›th sub-expression was not used in the match, or if ‹n› is greater than the number of sub-expressions in pattern, then \‹n› is replaced with the empty string.

To substitute a literal & or \, use \& and \\, respectively, in insert. A \$ in insert is equivalent to an empty sequence; this can be used to terminate a number ‹n› following \. If a \ in insert is followed by anything other than a digit, &, \, or $, then the \ by itself is treated as \0.

Note that the \ described in the previous paragraphs is a character or byte of input. To write such an input as a Racket string literal, an escaping \ is needed before the \. For example, the Racket constant "\\1" is \1.

Examples:

> (regexp-replace #rx"mi" "mi casa" "su")
"su casa"
> (regexp-replace #rx"mi" "mi casa" string-upcase)
"MI casa"
> (regexp-replace #rx"([Mm])i ([a-zA-Z]*)" "Mi Casa" "\\1y \\2")
"My Casa"
> (regexp-replace #rx"([Mm])i ([a-zA-Z]*)" "mi cerveza Mi Mi Mi"
"\\1y \\2")
"my cerveza Mi Mi Mi"
> (regexp-replace #rx"x" "12x4x6" "\\\\")
"12\\4x6"
> (display (regexp-replace #rx"x" "12x4x6" "\\\\"))
12\4x6

procedure
(regexp-replace* pattern
input
insert
[ start-pos
end-pos
input-prefix]) → (or/c string? bytes?)
  pattern : (or/c string? bytes? regexp? byte-regexp?)
  input : (or/c string? bytes?)
   insert :
(or/c string? bytes?
      ((string?) () #:rest (listof string?) . ->* . string?)
      ((bytes?) () #:rest (listof bytes?) . ->* . bytes?))
  start-pos : exact-nonnegative-integer? = 0
  end-pos : (or/c exact-nonnegative-integer? #f) = #f
  input-prefix : bytes? = #""

Like regexp-replace, except that every instance of pattern in input is replaced with insert, instead of just the first match. Only non-overlapping instances of pattern in input are replaced, so instances of pattern within inserted strings are not replaced recursively. Zero-length matches are treated the same as in regexp-match*.

The optional start-pos and end-pos arguments select a portion of input for matching; the default is the entire string or the stream up to an end-of-file.

Examples:

> (regexp-replace* #rx"([Mm])i ([a-zA-Z]*)" "mi cerveza Mi Mi Mi"
                   "\\1y \\2")
"my cerveza My Mi Mi"
> (regexp-replace* #rx"([Mm])i ([a-zA-Z]*)" "mi cerveza Mi Mi Mi"
                   (lambda (all one two)
                     (string-append (string-downcase one) "y"
                                    (string-upcase two))))
"myCERVEZA myMI Mi"
> (regexp-replace* #px"\\w" "hello world" string-upcase 0 5)
"HELLO world"
> (display (regexp-replace* #rx"x" "12x4x6" "\\\\"))
12\4\6

procedure
(regexp-replaces input replacements) → (or/c string? bytes?)
  input : (or/c string? bytes?)
   replacements :
(listof
(list/c (or/c string? bytes? regexp? byte-regexp?)
         (or/c string? bytes?
             ((string?) () #:rest (listof string?) . ->* . string?)
             ((bytes?) () #:rest (listof bytes?) . ->* . bytes?))))

Performs a chain of regexp-replace* operations, where each element in replacements specifies a replacement as a (list pattern replacement). The replacements are done in order, so later replacements can apply to previous insertions.

Examples:

> (regexp-replaces "zero-or-more?"
'([#rx"-" "_"] [#rx"(.*)\\?$" "is_\\1"]))
"is_zero_or_more"
> (regexp-replaces "zero-or-more?"
'([#rx"e" "o"] [#rx"o" "oo"]))
"zooroo-oor-mooroo?"

procedure
(regexp-replace-quote str) → string?
str : string?
(regexp-replace-quote bstr) → bytes?
bstr : bytes?

Produces a string suitable for use as the third argument to regexp-replace to insert the literal sequence of characters in str or bytes in bstr as a replacement. Concretely, every \ and & in str or bstr is protected by a quoting \.

Examples:

> (regexp-replace #rx"UT" "Go UT!" "A&M")
"Go AUTM!"
> (regexp-replace #rx"UT" "Go UT!" (regexp-replace-quote "A&M"))
"Go A&M!"

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1	Language Model
2	Notation for Documentation
3	Syntactic Forms
4	Datatypes
5	Structures
6	Classes and Objects
7	Units
8	Contracts
9	Pattern Matching
10	Control Flow
11	Concurrency and Parallelism
12	Macros
13	Input and Output
14	Reflection and Security
15	Operating System
16	Memory Management
17	Unsafe Operations
18	Running Racket
	Bibliography
	Index

4.1	Equality
4.2	Booleans
4.3	Numbers
4.4	Strings
4.5	Byte Strings
4.6	Characters
4.7	Symbols
4.8	Regular Expressions
4.9	Keywords
4.10	Pairs and Lists
4.11	Mutable Pairs and Lists
4.12	Vectors
4.13	Boxes
4.14	Hash Tables
4.15	Sequences and Streams
4.16	Dictionaries
4.17	Sets
4.18	Procedures
4.19	Void
4.20	Undefined

4.8.1	Regexp Syntax
4.8.2	Additional Syntactic Constraints
4.8.3	Regexp Constructors
4.8.4	Regexp Matching
4.8.5	Regexp Splitting
4.8.6	Regexp Substitution