• RegularExpression

• PythonLibraryReference, [http://www.python.org/doc/current/lib/module-re.html 4.2 re module]

• PythonLibraryReference, [http://www.python.org/doc/current/lib/re-syntax.html 4.2.1 Regular Expression Syntax]

Reference Diagrams

http://taoriver.net/img/for_pi/regex_characters.png

flags when compiling:

http://taoriver.net/img/for_pi/regex_flags.png

(All images PD, released by me, author, LionKimbro.)

Searching & Matching

You can search or match.

• search -- find something anywhere in the string, and return it

• match -- find something from the beginning of the string, and return it

You can also split on a pattern.

For example:

   1 import re
   2 split_up = re.split( r"(\(\([^)]+\)\))",
   3                      "This is a ((test)) of the ((emergency broadcasting station.))" )

...which produces:

["This is a ", "((test))", " of the ", "((emergency broadcasting station.))" ]

Compiling

If you use a regex a lot, compile it first.

Consider:

   1 import re
   2 match_obj=re.match( "<(.*?)>(.*?)</(.*?)>", "<h1>robot</h1>" )
   3 print mo.groups()

...which outputs: ('h1', 'robot', 'h1')

If you were going to do that match a lot, you could compile it, like so:

   1 import re
   2 match_re=re.compile( "<(.*?)>(.*?)</(.*?)>" )
   3 match_obj=match_re.match( "<h1>robot</h1>" )
   4 print mo.groups()

...which yields the same result.

I don't know how much faster compiled forms are than non-compiled forms.

Links

• [http://www.amk.ca/python/howto/regex/ Regular Expression HOWTO] - excellent Python-based regular expression tutorial, by [http://www.amk.ca/ A.M. Kuchling.]

For those interested in visualization, you may also be interested in a [http://www.ozonehouse.com/mark/blog/code/PeriodicTable.pdf periodic table of PERL operators.]

Discussion

I've made a couple of diagrams, which I've linked at the top of the page here.

I have SVG links as well; [http://taoriver.net/img/for_pi/regex_characters.svg first] and [http://taoriver.net/img/for_pi/regex_flags.svg second.]

Damn the [http://visual.wiki.taoriver.net/moin.cgi/LongImageIncorporationProcess LongImageIncorporationProcess!] Damn it to hell! We'd have a billion pretty pictures here, if tablets were cheap, and we had protocols and implementations for saving and loading straight to and from the wiki.

I have another diagram I'd like to make and place; It's the pattern (RegexObject) and match (MatchObject) API, visualized, and arranged dense. We'll see if I get around to drawing it, but it looks like no. Too much else to do.

-- LionKimbro DateTime(2004-12-28T08:38:12Z)

• I like the Venn diagram in this image. However, one part of the image is confusing. Where it refers to python strings, and "regex strings" (which are actually Python "raw" strings) and something called "match strings" ... what are these "match strings. -- JimD DateTime(2004-12-30T20:03:54Z)