Help with Grep - Apply Format for Line with X characters

Hi @Henri36655125at8n, because you tagged "scripting" here is a scripting approach. This is a first draft, because we don't know exactly what you want, eg. do you want *only* paragraphs with specific numbers of characters changed, like 4 and 20, or do you want every paragraph with 4 or more, or 20 or less, or ... ? My example script assumes the latter, which is probably wrong, but it's a starting point.

You configure the script in two ways: (1) by specifying a "base style name", eg. "Size Me" in my demo doc, and (2) by creating style variants for that style with the number at the end delimited by an underscore, eg. "Size Me_14" in my demo doc.

To see it working, you can open the attached demo.indd, which has the styles created.

- Mark

/**
 * Set paragraph styles based on number of characters in paragraph.
 * @author m1b
 * @discussion https://community.adobe.com/t5/indesign-discussions/help-with-grep-apply-format-for-line-with-x-characters/m-p/14548751
 */
function main() {

    var doc = app.activeDocument;

    setStylesByParagraphLength(doc, 'Size Me')

};
app.doScript(main, ScriptLanguage.JAVASCRIPT, undefined, UndoModes.ENTIRE_SCRIPT, 'Set Styles By Contents Length');

/**
 * Finds all paragraphs in `doc` that have a paragraph style
 * applied that has "size variants", and applies the
 * variant style based on the number of characters
 * in the paragraph.
 *
 * Example variants:
 *   Size Me      - the base style
 *   Size Me_4    - apply when paragraph length >= 4
 *   Size Me_10   - apply when paragraph length >= 10
 *   Size Me_15   - apply when paragraph length >= 15
 *   ...
 *
 * The nearest lower style is applied, so a paragraph
 * with 20 characters will get the Size Me_15 style,
 * and a paragraph with 14 characters will get the
 * Size Me_10 style.
 * @author m1b
 * @version 2024-04-11
 * @param {Document} doc - an Indesign Document.
 * @param {String} baseStyleName - the base style name.
 */
function setStylesByParagraphLength(doc, baseStyleName) {

    var styles = doc.allParagraphStyles;

    // the base style is the one with no "variant number"
    var baseStyle = getThing(styles, 'name', baseStyleName);

    if (undefined == baseStyle)
        return alert('Could not find paragraph style "' + baseStyleName + '".');

    // store variant styles in array indexed by their variant number
    var re = new RegExp('^' + baseStyleName + '_(\\d+)$'),
        numberedStyleVariants = [baseStyle];

    for (var i = 0, match, n = styles.length; i < n; i++) {

        match = styles[i].name.match(re);

        if (
            null === match
            || match.length < 2
        )
            continue;

        numberedStyleVariants[Number(match[1])] = styles[i];

    }

    if (0 === numberedStyleVariants.length)
        return alert('No numbered variants of "' + baseStyleName + '" were found, eg. "' + baseStyleName + '_14".');

    // find paragraphs set in the base style
    app.findGrepPreferences = NothingEnum.NOTHING;
    app.findGrepPreferences.appliedParagraphStyle = baseStyle;
    var found = doc.stories.everyItem().paragraphs.everyItem().findGrep();

    foundLoop:
    for (var i = found.length - 1, n, para; i >= 0; i--) {

        paragraphsLoop:
        for (var j = found[i].length - 1; j >= 0; j--) {

            para = found[i][j];

            // don't include the carriage-return
            n = para.length + ('\r' === para.characters[-1].contents ? 0 : 1);

            // apply the style variant number `n` (or next lowest)
            while (n--) {
                if (undefined != numberedStyleVariants[n]) {
                    para.appliedParagraphStyle = numberedStyleVariants[n];
                    continue paragraphsLoop;
                }
            }

        }

    }

};

/**
 * Returns a thing with matching property.
 * @param {Array|collection} things - the things to look through, eg. PageItems.
 * @param {String} key - the property name, eg. 'name'.
 * @param {*} value - the value to match.
 * @returns {*} - the thing.
 */
function getThing(things, key, value) {

    for (var i = 0; i < things.length; i++)
        if (things[i][key] == value)
            return things[i];

};

Edit: oops, forgot to add demo.indd.