Unexpected RegEx behavior in Delphi XE

Question

Delphi XE, using Delphi's own RegularExpressions unit. I'm attempting to correct some bad RTF code, where 'bookmark' tags cross the boundaries of a table cell. Seems simple enough. The code I'm using is below. Here's the general idea. Given this text ``` {\*\bkmkstart BM0}\plain\f0\fs24\cf0 ^\cell} ``` Look for a match to this pattern (there should be exactly one in the given text): ``` {\\*\bkmkstart BM0}\plain\f[0-9]\fs[0-9]+\cf[0-9] \^\cell} ``` When found, replace it with this (non-RegEx) string: ``` {\*\bkmkstart BM0}\plain\f0\fs24\cf0 ^{\*\bkmkend BM0}\plain\f0\fs24\cf0 \cell} ``` The expected results are that the first string should be replaced with the last string, eg: ``` {\*\bkmkstart BM0}\plain\f0\fs24\cf0 ^\cell} *becomes* {\*\bkmkstart BM0}\plain\f0\fs24\cf0 ^{\*\bkmkend BM0}\plain\f0\fs24\cf0 \cell} ``` However, the result I'm actually getting is this: ``` {\*\bkmkstart BM0}\plain{\*\bkmkstart bm0}\plain\f0\fs24\cf0 ^\cell}\fs24\cf0 ^{\*\bkmkend BM0}\plain{\*\bkmk…

Accepted Answer

You need to escape the `\` characters in your replacement string:

```
ReplaceWith = '{\*\bkmkstart BM0}\plain\f0\fs24\cf0 ^{\*\bkmkend BM0}\plain\f0\fs24\cf0 \cell}';
```

When you make this change the output is:

```
{\*\bkmkstart BM0}\plain\f0\fs24\cf0 ^{\*\bkmkend BM0}\plain\f0\fs24\cf0 \cell}
```

In fact, for your replacement string, you only need to escape the backslash in \f0 which, as it happens, appears twice. Personally I think it's just easier to escape the backslash indiscriminately.

By combining regular expressions and RTF you've mixed your own special backslash soup — tread carefully. Just be thankful you aren't using C or older versions of C++ that do not support raw strings. That backslash soup would be completely unpalatable!

Unexpected RegEx behavior in Delphi XE

Solution

Problem