Decoding a regex

Question

I am looking to decode a regular expression. Is there a way to check what the below regex means:

^(PC([Y\\d])|GC([Y\\d])|Y|\\d)\\d{4,5}$

Answer 1

You could use the Regular Expression Analyzer at http://www.myezapp.com/apps/dev/regexp/show.ws or http://www.debuggex.com/

---

^ = start of string
() = capturing groups
[] = character classes
\d = digit in 0-9
\\ = literal backslash
| = OR
{} = count of leading item
$ = end of string

Answer 2

Here is a break down of what is happening.

Regex: ^(PC([Y\\d])|GC([Y\\d])|Y|\\d)\\d{4,5}$

 1. ^        -  Beginning of line  
 2. (        -  Beginning of a capture group
 3. PC       -  Finds `PC` exactly  
 4. ([Y\\d]) -  Creates a capture group for a Y or a single digit (0-9)
 5. |        -  This is an OR statement
 6. GC       -  Finds `GC` exactly
 7. ([Y\\d]) -  Same as 4
 8. |        -  This is an OR statement
 9. Y        -  Finds `Y` exactly
10. |        -  This is an OR statement
11. \\d      -  This looks for a single digit (0-9)
12. )        -  End of capture group.  Lines 3-11 will be in this capture group
13. \\d{4,5} -  This will look any digit exactly 4 or 5 times
14. $        -  End of line

There are 3 capture groups in this:

1. (PC([Y\\d])|GC([Y\\d])|Y|\\d)
2. ([Y\\d])  (The first one)
3. ([Y\\d])  (The second one)

Here is a list of valid matches (any number would be found, I just used 123456 to show how many number places there can be):

• PCY1234
• PCY12345
• PCY1234
• PCY12345
• PC12345
• PC123456
• GC12345
• GC123456
• GCY1234
• GCY12345
• Y1234
• Y12345
• 12345
• 123456

Here is a link to RegExr with explanations of the capture groups for each match.

Also, the reason for the double \ in \\d is to escape the \ for Java. Not all languages need this, and from what I understand, there are some that need 3. If you notice in the RegExr above, I removed them so RegExr would correctly parse the Regex.