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https://github.com/zyedidia/sregx

A tool and library for using structural regular expressions.
https://github.com/zyedidia/sregx

golang regular-expressions structural-regex structural-regular-expressions

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A tool and library for using structural regular expressions.

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README 

        
# Structural Regular Expressions



[![Go Reference](https://pkg.go.dev/badge/github.com/zyedidia/sregx.svg)](https://pkg.go.dev/github.com/zyedidia/sregx)

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sregx is a package and tool for using structural regular expressions as described

by Rob Pike ([link](http://doc.cat-v.org/bell_labs/structural_regexps/)). sregx

provides a very simple Go package for creating structural regular expression

commands as well as a library for parsing and compiling sregx commands from the

text format used in Pike's description. A CLI tool for using structural regular

expressions is also provided in `./cmd/sregx`, allowing you to perform advanced

text manipulation from the command-line.



In a structural regular expression, regular expressions are composed using

commands to perform tasks like advanced search and replace. A command has

an input string and produces an output string. The following commands are

supported:



* `p`: prints the input string, and then returns the input string.

* `d`: returns the empty string.

* `c//`: returns the string ``.

* `s/
//`: returns a string where substrings matching the regular

  expression `
` have been replaced with ``.

* `g/
/`: if `
` matches the input, returns the result of ``

  evaluated on the input. Otherwise returns the input with no modification.

* `v/
/`: if `
` does not match the input, returns the result of

  `` evaluated on the input. Otherwise returns the input with no

  modification.

* `x/
/`: returns a string where all substrings matching the regular

  expression `
` have been replaced with the return value of `` applied

  to the particular substring.

* `y/
/`: returns a string where each part of the string that is not

  matched by `
` is replaced by applying `` to the particular

  unmatched string.

* `n[N:M]`: returns the application of `` to the input sliced from

  `[N:M)`. Accepts negative numbers to refer to offsets from the end of the

  input. Offsets are zero-indexed.

* `l[N:M]`: returns the application of `` to the input sliced from

  line `N` to line `M` (exclusive).  Assumes newlines are represented with the

  `\n` character. Accepts negative numbers to refer to offsets from the last

  line of the input. Lines are zero-indexed.

* `u//`: executes the shell command `` with the input as stdin and

  returns the resulting stdout of the command. Shell commands use a simple

  syntax where single or double quotes can be used to group arguments, and

  environment variables are accessible with `$`. This command is only directly

  available as part of the sregx CLI tool.



The commands `n[...]`, `l[...]`, and `u` are additions to the original

description of structural regular expressions.



The sregx tool also provides another augmentation to the original sregx description

from Pike: command pipelines. A command may be given as ` |  | ...`

where the input of each command is the output of the previous one.



### Examples



Most of these examples are from Pike's description, so you can look there for

more detailed explanation. Since `p` is the only command that prints,

technically you must append `| p` to commands that search and replace, because

otherwise nothing will be printed. However, since you will probably forget to

do this, the sregx tool will print the result of the final command before

terminating if there were no uses of `p` anywhere within the command. Thus when

using the CLI tool you can omit the `| p` in the following commands and still

see the result.



Print all lines that contain "string":



```

x/.*\n/ g/string/p

```



Delete all occurrences of "string" and print the result:



```

x/string/d | p

```



Replace all occurrences of "foo" with "bar" in the range of lines 5-10

(zero-indexed):



```

l[5:10]s/foo/bar/ | p

```



Print all lines containing "rob" but not "robot":



```

x/.*\n/ g/rob/ v/robot/p

```



Capitalize all occurrences of the word "i":



```

x/[A-Za-z]+/ g/i/ v/../ c/I/ | p

```



or (more simply)



```

x/[A-Za-z]+/ g/^i$/ c/I/ | p

```



Print the last line of every paragraph that begins with "foo", where a

paragraph is defined as text with no empty lines:



```

x/(.+\n)+/ g/^foo/ l[-2:-1]p

```



Change all occurrences of the complete word "foo" to "bar" except those

occurring in double or single quoted strings:



```

y/".*"/ y/'.*'/ x/[a-zA-Z]+/ g/^foo$/ c/bar/ | p

```



Replace the complete word "TODAY" with the current date:



```

x/[A-Z]+/ g/^TODAY$/ u/date/ | p

```



Capitalize all words:



```

x/[a-zA-Z]+/ x/^./ u/tr a-z A-Z/ | p

```



Note: it is highly recommended when using the CLI tool that you enclose

expressions in single or double quotes to prevent your shell from interpreting

special characters.



## Installation



There are three ways to install `sregx`.



1. Download the prebuilt binary from the releases page (comes with man file).



2. Install from source:



```

git clone https://github.com/zyedidia/sregx

cd sregx

make build # or make install to install to $GOBIN

```



3. Install with `go get` (version info will be missing):



```

go get github.com/zyedidia/sregx/cmd/sregx

```



### Usage



To use the CLI tool, first pass the expression and then the input file. If no

file is given, stdin will be used. Here is an example to capitalize all

occurrences of the word 'i' in `file.txt`:



```

sregx 'x/[A-Za-z]+/ g/^i$/ c/I/' file.txt

```



The tool tries to provide high quality error messages when you make a mistake

in the expression syntax.



## Base library



The base library is very simple and small (roughly 100 lines of code). In fact,

it is surprisingly simple and elegant for something that can provide such

powerful text manipulation, and I recommend reading the code if you are

interested. Each type of command may be manually created directly in tree form.

See the Go documentation for details.



## Syntax library



The syntax library supports parsing and compiling a string into a structural

regular expression command. The syntax follows certain rules, such as using "/"

as a delimiter. The backslash (`\`) may be used to escape `/` or `\`, or to

create special characters such as `\n`, `\r`, or `\t`. The syntax also supports

specifying arbitrary bytes using octal, for example `\14`. Regular expressions

use the Go syntax described [here](https://golang.org/pkg/regexp/syntax/).



# Future Work



Here are some ideas for some features that could be implemented in the future.



* Internal manipulation language. Currently the `u` command runs shell

  commands. This is very flexible but can be costly because a new process is

  run to perform each transformation. For better performance we could provide a

  small language that has some string manipulation functions like `toupper`. A

  good candidate for this language would be Lua. This would also improve

  Windows support since most Windows environments lack utilities like `tr`.

* Different regex engine. The Go regex engine is pretty good, but isn't

  especially performant. We could switch to Oniguruma (see the `oniguruma`

  branch), although this would mean using cgo.

* Structural PEGs. Use PEGs instead of regular expressions.