Regular expression explained with words

Question

I need a web page or Windows application that takes a regular expression and explains it to me in text what it does.

Do you have any suggestions for such an application?

Answer 1

The Regular Expression Workbench explains regular expressions in pseudocode (via the "Interpret" button).

Answer 2

I quite like Eric Gunnerson's Regex Workbench. It's simple, free, and does this.

Answer 3

YAPE::Regex::Explain comes with a utility named explain that does this. I believe there is also a builit-in layer in later versions of Perl that also does something similar.

These tools are generally not very useful because, like bad comments, they tell you what a regex does, not why it is doing it.

Answer 4

RegEx Buddy has an analyze feature.

And is recommended by Jeff.

Answer 5

Python has built-in support for this, using re.DEBUG or the sre_parse module:

>>> import re
>>> r = re.compile('(h\S+) (wo\S+)', re.DEBUG)
subpattern 1
  literal 104
  max_repeat 1 65535
    in
      category category_not_space
literal 32
subpattern 2
  literal 119
  literal 111
  max_repeat 1 65535
    in
      category category_not_space

>>> import sre_parse
>>> sre_parse.parse('(h\S+) (wo\S+)')
[('subpattern', (1, [('literal', 104), ('max_repeat', (1, 65535, [('in', [('category', 'category_not_space')])]))])), ('literal', 32), ('subpattern', (2, [('literal', 119), ('literal', 111), ('max_repeat', (1, 65535, [('in', [('category', 'category_not_space')])]))]))]

But the output of Regex buddy looks a bit nicer, I confess... However, it should not be too hard to transform the output of sre_parse.parse into something similar.

Answer 6

Edi Weitz's regex coach does this, and more.

Answer 7

Expresso is a great one I use all the time.

Answer 8

Perl has a wonderful Regex parser that will explain a regex in detail, using plain English as far as possible.

YAPE::Regex::Explain

See this answer for an example.

Answer 9

Perl has a built in way to do something similar:

perl -Mre=debug -e"'aaabbbccdd'=~/a[b]+c?/"

# at Compile time
Compiling REx "a[b]+c?"
Final program:
   1: EXACT <a> (3)
   3: PLUS (6)
   4:   EXACT <b> (0)
   6: CURLY {0,1} (10)
   8:   EXACT <c> (0)
  10: END (0)
anchored "ab" at 0 floating "b" at 1..2147483647 (checking anchored) minlen 2 

# at run time
Guessing start of match in sv for REx "a[b]+c?" against "aaabbbccdd"
Found anchored substr "ab" at offset 2...
Found floating substr "b" at offset 3...
Starting position does not contradict "/^/m"...
Guessed: match at offset 2
Matching REx "a[b]+c?" against "abbbccdd"
   2 <aa> <abbbccdd>         |  1:EXACT <a>(3)
   3 <aaa> <bbbccdd>         |  3:PLUS(6)
                                  EXACT <b> can match 3 times out of 2147483647...
   6 <aaabbb> <ccdd>         |  6:  CURLY {0,1}(10)
                                    EXACT <c> can match 1 times out of 1...
   7 <aaabbbc> <cdd>         | 10:    END(0)
Match successful!

# at exit time
Freeing REx: "a[b]+c?"

use re 'debug';

Answer 10

strfriend is an online regex viewer and works pretty well.

Here are some examples (via tr.im; SO borks on these links):

• Email
• Credit Card
• HTML Tags