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Regular Expressions

The ANSI Common Lisp standard does not include facilities for regular expressions, but a couple of libraries exist for this task, for instance: cl-ppcre.

See also the respective Cliki: regexp page for more links.

Note that some CL implementations include regexp facilities, notably CLISP and ALLEGRO CL. If in doubt, check your manual or ask your vendor.

The description provided below is far from complete, so don't forget to check the reference manual that comes along with the CL-PPCRE library.

PPCRE

Using PPCRE

CL-PPCRE (abbreviation for Portable Perl-compatible regular expressions) is a portable regular expression library for Common Lisp with a broad set of features and good performance. It has been ported to a number of Common Lisp implementations and can be easily installed (or added as a dependency) via Quicklisp:

(ql:quickload "cl-ppcre")

Basic operations with the CL-PPCRE library functions are described below.

Looking for matching patterns

The scan function tries to match the given pattern and on success returns four multiple-values values - the start of the match, the end of the match, and two arrays denoting the beginnings and ends of register matches. On failure returns NIL.

A regular expression pattern can be compiled with the create-scanner function call. A "scanner" will be created that can be used by other functions.

For example:

(let ((ptrn (ppcre:create-scanner "(a)*b")))
  (ppcre:scan ptrn "xaaabd"))

will yield the same results as:

(ppcre:scan "(a)*b" "xaaabd")

but will require less time for repeated scan calls as parsing the expression and compiling it is done only once.

Extracting information

CL-PPCRE provides a several ways to extract matching fragments, among them: the scan-to-strings and register-groups-bind functions.

The scan-to-strings function is similar to scan but returns substrings of target-string instead of positions. This function returns two values on success: the whole match as a string plus an array of substrings (or NILs) corresponding to the matched registers.

The register-groups-bind function tries to match the given pattern against the target string and binds matching fragments with the given variables.

(ppcre:register-groups-bind (first second third fourth)
      ("((a)|(b)|(c))+" "abababc" :sharedp t)
    (list first second third fourth))
;; => ("c" "a" "b" "c")

CL-PPCRE also provides a shortcut for calling a function before assigning the matching fragment to the variable:

(ppcre:register-groups-bind (fname lname (#'parse-integer date month year))
      ("(\\w+)\\s+(\\w+)\\s+(\\d{1,2})\\.(\\d{1,2})\\.(\\d{4})" "Frank Zappa 21.12.1940")
    (list fname lname (encode-universal-time 0 0 0 date month year 0)))
;; => ("Frank" "Zappa" 1292889600)

Syntactic sugar

It might be more convenient to use CL-PPCRE with the CL-INTERPOL library. CL-INTERPOL is a library for Common Lisp which modifies the reader in a way that introduces interpolation within strings similar to Perl, Scala, or Unix Shell scripts.

In addition to loading the CL-INTERPOL library, initialization call must be made to properly configure the Lisp reader. This is accomplished by either calling the enable-interpol-syntax function from the REPL or placing that call in the source file before using any of its features:

(interpol:enable-interpol-syntax)

A lot more syntax sugar is introduced by the cl21 project but in a somewhat more intrusive way.