Is there a way to "recall" a char sequence already matched in the regex itself?

Question

The regex I'm searching has the following constraints: - it starts with "//" - then "[" a non number sequence (called delimiter in this list) and "]" - next line "\n" - "[" 0 or more number separated by the delimiter previously found "]". For example the following text matches the regex: ``` //[*#*] [1*#*34*#*64] ``` and the following text doesn't match the regex: ``` //[*#*] [1#34#64] ``` because the delimiter is not the same matched in the first row The regex I currently create is ``` ^//$$(\D)+$$\n$$[(\d)+(\D)+]*(\d)+$$$|^//$$(\D)+$$\n$|^//$$(\D)+$$\n$$(\d)+$$$ ``` but obviously this regex match with both previous examples. Is there a way to "recall" a char sequence already matched in the regex itself?

Accepted Answer

You need something called back-reference (a very good tutorial here).

Use this regex in Python:

```
r'^//$$([^$$]+)\]\n$$\d+(\1\d+)*$$'
```

Sample run:

```
>>> string = """//[*#*]
... [1*#*34*#*64]"""
>>> print re.search(r'^//$$([^$$]+)\]\n$$\d+(\1\d+)*$$',string).group(0)
//[*#*]
[1*#*34*#*64]
```

will match your string in Python.

Debuggex Demo

Is there a way to "recall" a char sequence already matched in the regex itself?

Solution

Problem